Lecture Notes: Introduction to Metabolism and Glycolysis PDF

Intro duc tio n to Me tabo lis m and Glyc o lys is 8 I. INTRODUCTION TO METABOLIS M The pro duc t o f o ne...

Intro duc tio n to Me tabo lis m and Glyc o lys is 8 I. INTRODUCTION TO METABOLIS M The pro duc t o f o ne re ac tio n is the s ubs trate o f the In Chapter 5, individua l enzymic reactions we re analyzed in an effort to s ubs e que nt re ac tio n. explain the me cha nisms of ca ta lys is. Howe ver, in ce lls, the s e re a ctions rarely occur in isolation but, rather, are organized into multistep sequences Glucos e 6-P Glucos e Gluc called pathways, such as that of glycolysis (Figure 8.1). In a pathway, the product of one reaction serves as the substrate of the subsequent reaction. Fructos e 6-P Different pathways can also intersect, forming an integrated and purposeful Fructos e 1,6-bis phos pha te network of chemical reactions. These are collectively called metabolism, which is the sum of all the chemical changes occurring in a cell, a tissue, Glyce ra lde hyde 3-P Dihydroxy- or the body. Most pathways can be classified as either catabolic (degra- a ce tone -P da tive ) or ana bolic (s ynthe tic). Ca ta bolic re actions bre ak down comple x 1,3-Bis phos phoglyce ra te molecules, such as proteins, polysaccharides, and lipids, to a few simple 3-P hos phoglyce ra te molecules (for example, CO 2, NH3 [ammonia], and H2O). Anabolic path- ways form complex end products from simple precursors, for example, the 2-P hos phoglyce ra te synthesis of the polysaccharide, glycogen, from glucose. [Note: Pathways that regenerate a component are called cycles.] In the following chapters, P hos phoe nolpyruva te this te xt focuse s on the centra l metabolic pa thways tha t a re involve d in La cta te P yruva te synthesizing and degrading carbohydrates, lipids, and amino acids. A. Me tabo lic map It is conve nie nt to inve s tiga te me ta bolis m by e xa mining its compone nt Fig ure 8.1 pa thwa ys. Ea ch pa thwa y is compos e d of multie nzyme s e que nce s , Glycolys is , a n e xa mple of a me ta bolic pa thwa y. [Note : P yruva te a nd e a ch e nzyme , in turn, ma y e xhibit importa nt ca ta lytic or re gula - to phos phoe nolpyruva te re quire s tory fe a ture s. To provide the re a de r with the “big picture ,” a me ta bolic two re a ctions.] Curve d re a ction a rrows ma p conta ining the importa nt ce ntra l pa thwa ys of ene rgy me ta bolis m ( ) indica te forwa rd a nd re ve rs e is pre s e nte d in Figure 8.2. This ma p is us e ful in tra cing conne ctions re a ctions tha t a re ca ta lyze d by be twe e n pa thwa ys , vis ua lizing the purpos e ful “move me nt” of me ta - diffe re nt e nzyme s.P = phos pha te. bolic inte rme dia te s , a nd de picting the e ffe ct on the flow of inte rme di- a te s if a pa thwa y is blocke d (for e xa mple , by a drug or a n inhe rite d de ficie ncy of a n e nzyme ). Throughout the ne xt thre e units of this book, e a ch pa thwa y unde r dis cus s ion will be re pe a te dly fe a ture d a s pa rt of the ma jor me ta bolic ma p s hown in Figure 8.2. B. Catabo lic pathways Ca ta bolic re a ctions s e rve to ca pture che mica l e ne rgy in the form of a de nos ine triphos pha te (ATP ) from the de gra da tion of e ne rgy-rich fue l mole cule s. Ca ta bolis m a ls o a llows mole cule s in the die t (or nutri- e nt mole cule s s tore d in ce lls ) to be conve rte d into building blocks ne e de d for the s ynthe s is of comple x mole cule s. Energy ge ne ra tion by 91 t ahir99-VRG & vip.p ersianss.ir 92 8. Introduction to Me ta bolis m a nd Glycolys is 6-P g luc o nate Glyc o g e n Galac to s e Ribulo s e 5-P 6-P g luc o no lac to ne UDP-Gluc o s e Galac to s e 1-P Pe nto s e Ribo s e 5-P pho s phate Gluc o s e 1-P UDP-Galac to s e pathway Xylulo s e 5-P Gluc o s e 6-P Gluc o s e S e do he ptulo s e 7-P Fruc to s e 6-P Fruc to s e Erythro s e 4-P Fruc to s e 1,6-bis -P Glyc e ralde hyde Fruc to s e 1-P Glyc e ralde hyde 3-P Glyc e ralde hyde 3-P Dihydro xyac e to ne -P Glyc o lys is 1,3-Bis pho s pho g lyc e rate Glyc e ro l-P Glyc e ro l 3-Pho s pho g lyc e rate Triac ylg lyc e ro l 2-Pho s pho g lyc e rate Fatty ac yl Co A Fatty ac ids Alanine Pho s pho e no lpyruvate Triac ylg lyc e ro l Cys te ine s ynthe s is and Glyc ine de g radatio n S e rine Lac tate Pyruvate Malo nyl Co A Thre o nine CO2 Le uc ine CO2 Phe nylalanine NH3 CO2 Tyro s ine Ac e tyl Co A Ac e to ac e tate Trypto phan As parag ine Lys ine Is o le uc ine Carbamo yl-P β-Hydro xybutyrate As partate Oxalo ac e tate Citrate Citrulline Malate Tric arbo xylic Is o c itrate Glutamine Arg inino s uc c inate ac id (TCA) CO2 c yc le Pro line Ornithine Ure a Fumarate α-Ke to g lutarate Glutamate His tidine c yc le Arg inine CO2 Arg inine S uc c inate S uc c inyl Co A Me thylmalo nyl Co A Ure a Is o le uc ine Pro pio nyl Co A Phe nylalanine Me thio nine Ac e tyl Co A Tyro s ine Valine Fatty ac yl Co A Thre o nine (o dd numbe r o f c arbo ns ) Fig ure 8.2 Importa nt re a ctions of inte rme dia ry me ta bolis m. S e ve ra l importa nt pa thwa ys to be dis cus s e d in la te r cha pte rs a re highlighte d. Curve d re a ction a rrows ( ) indica te forwa rd a nd re ve rs e re a ctions tha t a re ca ta lyze d by diffe re nt e nzyme s. The s tra ight a rrows ( ) indica te forwa rd a nd re ve rs e re a ctions tha t a re ca ta lyze d by the s a me e nzyme. Blue te xt = inte rme dia te s of ca rbohydra te me ta bolis m; brown text = inte rme dia te s of lipid me ta bolis m; green text = inte rme dia te s of prote in me ta bolis m. UDP = uridine diphos pha te ; P = phos pha te ; CoA = coe nzyme A. t ahir99-VRG & vip.p ersianss.ir II. Re gula tion of Me ta bolis m 93 S tag e I: Pro te ins Carbo hydrate s Fats Hydro lys is o f c o mple x mo le c ule s to the ir c o mpo ne nt building blo c ks Amino Mo no s ac c haride s Glyc e ro l, ac ids fatty ac ids S tag e II: Co nve rs io n o f building blo c ks to ac e tyl Co A (o r o the r s imple inte rme diate s ) Ac e tyl Co A S tag e III: ATP TCA Oxidatio n o f ac e tyl Co A; o xidative pho s pho rylatio n c yc le CO2 Fig ure 8.3 Thre e s ta ge s of ca ta bolis m. CoA = coe nzyme A; TCA = trica rboxylic a cid. de gra da tion of comple x mole cule s occurs in thre e s ta ge s a s s hown in Figure 8.3. [Note : Ca ta bolic pa thwa ys a re typica lly oxida tive , a nd re quire oxidize d coe nzyme s s uch a s nicotina mide a de nine dinucle o- tide (NAD+).] 1. Hydro lys is o f c o mple x mo le c ule s : In the firs t s ta ge , comple x mole cule s a re broke n down into the ir compone nt building blocks. For e xa mple , prote ins a re de gra de d to a mino a cids , polys a ccha - ride s to monos a ccha ride s , a nd fa ts (tria cylglyce rols ) to fre e fa tty a cids a nd glyce rol. 2. Conve rs io n of building bloc ks to s imple inte rme diate s : In the s e cond s ta ge , the s e dive rs e building blocks a re furthe r de gra de d to Ene rg y-yie lding Co mple x a ce tyl coe nzyme A (CoA) a nd a fe w othe r s imple mole cule s. S ome nutrie nts mo le c ule s e ne rgy is ca pture d a s ATP, but the a mount is s ma ll compa re d with Pro te ins the e ne rgy produce d during the third s ta ge of ca ta bolis m. Carbo hydrate s Po lys ac c haride s Fats Lipids 3. Oxidatio n o f ac e tyl c o e nzyme A: The trica rboxylic a cid (TCA) Pro te ins Nuc le ic ac ids cycle (s e e p. 109) is the fina l common pa thwa y in the oxida tion of fue l mole cule s tha t produce a ce tyl CoA. Oxida tion of a ce tyl C A CoA ge ne ra te s la rge a mounts of ATP via oxida tive phos phoryla - A Che mic al N T e ne rg y A tion a s e le ctrons flow from NADH a nd fla vin a de nine dinucle otide A B ATP B (FADH2 ) to oxyge n (s e e p. 73). O NADH O L L I I C. Anabo lic pathways S M S M Ene rg y-po o r Pre c urs o r Ana bolic re a ctions combine s ma ll mole cule s , s uch a s a mino a cids , e nd pro duc ts mo le c ule s to form comple x mole cule s s uch a s prote ins (Figure 8.4). Ana bolic Amino ac ids re a ctions re quire e ne rgy (a re e nde rgonic), which is ge ne ra lly pro- CO2 S ug ars vide d by the hydrolys is of ATP to a de nos ine diphos pha te (ADP ) a nd H2 O Fatty ac ids inorga nic phos pha te (P i). Ana bolic re a ctions ofte n involve che mica l Nitro g e no us NH3 bas e s re ductions in which the re ducing powe r is mos t fre que ntly provide d by the e le ctron donor NADP H (s e e p. 147). Note tha t ca ta bolis m is a conve rge nt proce s s (tha t is , a wide va rie ty of mole cule s a re tra ns - Fig ure 8.4 forme d into a fe w common e nd products ). By contra s t, a na bolis m is Compa ris on of ca ta bolic a nd a dive rge nt proce s s in which a fe w bios ynthe tic pre curs ors form a a na bolic pa thwa ys. ATP = a de nos ine triphos pha te ; NADH = nicotina mide wide va rie ty of polyme ric, or comple x, products. a de nine dinucle otide. II. REGULATION OF METABOLIS M The pa thwa ys of me ta bolis m mus t be coordina te d s o tha t the produc- tion of e ne rgy or the s ynthe s is of e nd products me e ts the ne e ds of the t ahir99-VRG & vip.p ersianss.ir 94 8. Introduction to Me ta bolis m a nd Glycolys is S ynaptic s ig naling ce ll. Furthe rmore , individua l ce lls do not function in is ola tion but, ra the r, Targ e t a re pa rt of a community of inte ra cting tis s ue s. Thus , a s ophis tica te d c e ll communica tion s ys te m ha s e volve d to coordina te the functions of the body. Re gula tory s igna ls tha t inform a n individua l ce ll of the me ta bolic s ta te of the body a s a whole include hormone s , ne urotra ns mitte rs , a nd Ne rve the a va ila bility of nutrie nts. The s e , in turn, influe nce s igna ls ge ne ra te d c e ll Ne uro - trans mitte r within the ce ll (Figure 8.5). Endo c rine s ig naling A. Intrac e llular c o mmunic atio n The ra te of a me ta bolic pa thwa y ca n re s pond to re gula tory s igna ls Hormo ne tha t a ris e from within the ce ll. For e xa mple , the ra te of a pa thwa y Targ e t ma y be influe nce d by the a va ila bility of s ubs tra te s , product inhibition, c e ll or a lte ra tions in the le ve ls of a llos te ric a ctiva tors or inhibitors. The s e Blo o d ve s s e l intra ce llula r s igna ls typica lly e licit ra pid re s pons e s , a nd a re importa nt for the mome nt-to-mome nt re gula tion of me ta bolis m. Dire c t c o ntac t B. Inte rc e llular c o mmunic atio n Gap junc tio n The ability to respond to intercellular signals is essential for the devel- opme nt a nd s urviva l of orga nis ms. Signa ling be twe e n ce lls provide s for long-ra nge inte gra tion of me ta bolis m a nd us ua lly re s ults in a S ig naling Targ e t re s pons e, s uch a s a cha nge in ge ne expre ss ion, tha t is slowe r tha n c e ll c e lls is s e e n with intra ce llula r s igna ls. Communica tion be twe e n ce lls ca n be mediated, for example, by surface-to-surface contact and, in some tissues, by formation of ga p junctions , allowing dire ct communica tion Fig ure 8.5 between the cytoplasms of adjacent cells. However, for energy metab- S ome commonly us e d me cha nis ms olism, the most importa nt route of communication is chemical signa l- for tra ns mis s ion of re gula tory s igna ls be twe e n ce lls. ing be twe e n ce lls by bloodborne hormone s or by ne urotra ns mitte rs. C. S e c o nd me s s e ng e r s ys te ms Hormone s or ne urotra ns mitte rs ca n be thought of a s s igna ls a nd the ir re ce ptors a s s igna l de te ctors. Ea ch compone nt s e rve s a s a link in the communica tion be twe e n e xtra ce llula r e ve nts a nd che mica l cha nge s within the ce ll. Ma ny re ce ptors s igna l the ir re cognition of a bound liga nd by initia ting a s e rie s of re a ctions tha t ultima te ly re s ult in a s pe cific intra ce llula r re s pons e. “S e cond me s s e nge r” mole cule s , s o na me d be ca us e the y inte rve ne be twe e n the origina l me s s e nge r (the ne urotra ns mitte r or hormone ) a nd the ultima te e ffe ct on the ce ll, a re The e xtrac e llular do main c o ntains pa rt of the ca s ca de of e ve nts tha t tra ns la te s (tra ns duce s ) hormone the binding s ite fo r a lig and or ne urotra ns mitte r binding into a ce llula r re s pons e. Two of the mos t (a ho rmo ne o r ne uro trans mitte r). wide ly re cognize d s e cond me s s e nge r s ys te ms a re the ca lcium/ phos pha tidylinos itol s ys te m (s e e p. 205) a nd the a de nylyl cycla s e (a de nyla te cycla s e ) s ys te m, which is pa rticula rly importa nt in re gula t- ing the pa thwa ys of inte rme dia ry me ta bolis m. D. Ade nylyl c yc las e The re cognition of a che mica l s igna l by s ome pla s ma (ce ll) me m- bra ne re ce ptors , s uch a s the β- a nd α 2 -a dre ne rgic re ce ptors , trig- ge rs e ithe r a n incre a s e or a de cre a s e in the a ctivity of a de nylyl The intrac e llular do main inte rac ts cycla s e (AC). This is a me mbra ne -bound e nzyme tha t conve rts ATP No te the s e ve n with G pro te ins. trans me mbrane to 3',5'-a de nos ine monophos pha te (commonly ca lle d cyclic AMP , α he llic e s. or cAMP ). The che mica l s igna ls a re mos t ofte n hormone s or ne u- rotra ns mitte rs , e a ch of which binds to a unique type of me mbra ne Fig ure 8.6 re ce ptor. The re fore , tis s ue s tha t re s pond to more tha n one che mica l S tructure of a typica l G prote in– s igna l mus t ha ve s e ve ra l diffe re nt re ce ptors , e a ch of which ca n be couple d re ce ptor of the pla s ma linke d to AC. The s e re ce ptors , known a s G prote in–couple d re ce p- me mbra ne. tors (GP CRs ), a re cha ra cte rize d by a n e xtra ce llula r liga nd-binding t ahir99-VRG & vip.p ersianss.ir II. Re gula tion of Me ta bolis m 95 doma in, s e ve n tra ns me mbra ne α he lice s , a nd a n intra ce llula r doma in Uno c c upie d re c e pto r do e s tha t inte ra cts with G prote ins (Figure 8.6). 1 no t inte rac t with Gs pro te in. Extra - Ho rmo ne o r ne uro - 1. Guano s ine tripho s phate –de pe nde nt re g ulato ry pro te ins : The ce llula r trans mitte r effect of the activated, occupied GPCR on second messenger for- space Ce ll Gs pro te in me mbra ne mation is not direct but, rather, is mediated by spe cialized trime ric with bo und GDP proteins (α, β, and γ subunits) of the ce ll membrane. These proteins, re fe rre d to a s G prote ins be ca us e the α s ubunit binds gua nine Re c e pto r nucleotides (GTP and GDP), form a link in the chain of communica- tion betwe en the receptor and AC. In the ina ctive form of a G pro- β γ tein, the α-subunit is bound to GDP (Figure 8.7). Binding of ligand α ca uses a conformational change in the receptor, triggering repla ce- Cytos ol Inac tive a d e n ylyl GDP c yc la s e ment of this GDP with GTP. The GTP-bound form of the α s ubunit dissociate s from the βγ subunits and moves to AC, which is thereby Oc c upie d re c e pto r c hang e s activate d. Ma ny mole cules of active Gα prote in are formed by one 2 s hape and inte rac ts with α s ubunit o f Gs pro te in. Gs pro te in activated re ce ptor. [Note : The a bility of a hormone or ne urotra ns- re le as e s GDP and binds GTP. mitter to stimulate or inhibit AC de pends on the type of Gα protein that is linke d to the rece ptor. One type, designated G s , stimula tes AC, whe rea s a nother type, designated G i, inhibits the enzyme (not s hown in Figure 8.7).] The a ctions of the Gα –GTP comple x a re short-lived because Gα has an inherent GTPa se a ctivity, re sulting in β γ the rapid hydrolysis of GTP to GDP. This causes inactiva tion of the Gα, its dissociation from AC, and re association with the βγ dimer. α GTP Inac tive a d e n ylyl GTP GDP c yc la s e Toxins from Vibrio chole ra e (chole ra ) a nd α S ubunit o f Gs pro te in Borde te lla pe rtus s is (whooping cough) ca us e ina p- 3 dis s o c iate s fro m βγ and ac tivate s a d e n ylyl c yc la s e. propria te a ctiva tion of a de nylyl cycla s e through cova le nt modifica tion (ADP -ribos yla tion) of diffe r- Ac tive a d e n ylyl e nt G prote ins. With chole ra , the GTPa s e a ctivity c yc la s e of Gα s is inhibite d in inte s tina l ce lls. With whooping cough, Gα i is ina ctiva te d in re s pira tory-tra ct ce lls. ATP β γ 2. Pro te in kinas e s : The ne xt ke y link in the cAMP s e cond me s s e n- α c AMP + PP i ge r s ys te m is the a ctiva tion by cAMP of a fa mily of e nzyme s ca lle d GTP cAMP -de pe nde nt prote in kina s e s s uch a s prote in kina s e A (Figure Whe n ho rmo ne is no lo ng e r 8.8). cAMP a ctiva te s prote in kina s e A by binding to its two re gula - 4 pre s e nt, the re c e pto r re ve rts to re s ting s tate. GTP o n the tory s ubunits , ca us ing the re le a s e of two a ctive , ca ta lytic s ubunits. α s ubunit is hydro lyze d to GDP, The a ctive s ubunits ca ta lyze the tra ns fe r of phos pha te from ATP α re jo ins βγ, and a d e n ylyl c yc la s e is de ac tivate d. to s pe cific s e rine or thre onine re s idue s of prote in s ubs tra te s. The phos phoryla te d prote ins ma y a ct dire ctly on the ce ll’s ion cha n- ne ls or, if e nzyme s , ma y be come a ctiva te d or inhibite d. P rote in kina s e A ca n a ls o phos phoryla te prote ins tha t bind to DNA, ca us - ing cha nge s in ge ne e xpre s s ion (s e e p. 456). [Note : S e ve ra l type s β γ α Inac tive of prote in kina s e s a re not cAMP de pe nde nt, for e xa mple , prote in a d e n ylyl kina s e C de s cribe d on p. 205.] GDP c yc la s e Pi 3. Dephos phorylation of proteins : The phosphate groups added to proteins by protein kinases a re removed by protein phosphatases, Fig ure 8.7 e nzyme s tha t hydrolytica lly cle a ve phos pha te e s te rs (s e e Figure The re cognition of che mica l s igna ls 8.8). This ensures tha t changes in protein activity induced by phos- by ce rta in me mbra ne re ce ptors phorylation a re not permanent. trigge rs a n incre a s e (or, le s s ofte n, a de cre a s e ) in the a ctivity of 4. Hydrolys is of cyc lic ade nos ine monophos phate : cAMP is ra p- a de nylyl cycla s e. GDP = gua nos ine idly hydrolyze d to 5'-AMP by cAMP phos phodie s te ra s e , one of a diphos pha te ; GTP = gua nos ine family of enzymes that cleave the cyclic 3',5'-phosphodiester bond. triphos pha te ; cAMP = cyclic AMP. t ahir99-VRG & vip.p ersianss.ir 96 8. Introduction to Me ta bolis m a nd Glycolys is 5'-AMP is not a n intra ce llula r s igna ling mole cule. The re fore , the Re g ulato ry Catalytic effects of neurotransmitter- or hormone-mediated increases of cAMP R C s ubunits s ubunits are rapidly terminated if the extracellular signal is removed. [Note: R C P hos phodie s te ra s e is inhibite d by the me thylxa nthine de riva tive , c AMP -d e p e n d e n t p ro te in kin a s e A caffeine.] Ade nylyl cycla s e ATP c AMP ( ) III. OVERVIEW OF GLYCOLYS IS The glycolytic pa thwa y is e mploye d by a ll tis s ue s for the oxida tion of C R glucos e to provide e ne rgy (in the form of ATP ) a nd inte rme dia te s for + othe r me ta bolic pa thwa ys. Glycolys is is a t the hub of ca rbohydra te R C me ta bolis m be ca us e virtua lly a ll s uga rs , whe the r a ris ing from the die t or from ca ta bolic re a ctions in the body, ca n ultima te ly be conve rte d to C P glucos e (Figure 8.9A). Pyruva te is the e nd product of glycolys is in ce lls Active ca ta lytic unit with mitochondria a nd a n a de qua te s upply of oxyge n. This s e rie s of te n of prote in kina se Pro te in Pho s pho rylate d re a ctions is ca lle d a e robic glycolys is be ca us e oxyge n is re quire d to re oxi- s ubs trate pro te in dize the NADH forme d during the oxida tion of glyce ra lde hyde 3-phos - H2 O pha te (Figure 8.9B). Ae robic glycolys is s e ts the s ta ge for the oxida tive ATP ADP P rote in phos pha ta s e de ca rboxyla tion of pyruva te to a ce tyl CoA, a ma jor fue l of the TCA cycle. Alte rna tive ly, pyruva te is re duce d to la cta te a s NADH is oxidize d to NAD+ Pi (Figure 8.9C). This conve rs ion of glucos e to la cta te is ca lle d a na e ro- bic glycolys is be ca us e it ca n occur without the pa rticipa tion of oxyge n. Ana e robic glycolys is a llows the production of ATP in tis s ue s tha t la ck INTRACELLULAR De pho s pho rylate d mitochondria (for e xa mple , re d blood ce lls a nd pa rts of the e ye ) or in ce lls EFFECTS pro te in de prive d of s ufficie nt oxyge n. Fig ure 8.8 IV. TRANS PORT OF GLUCOS E INTO CELLS Actions of cyclic AMP (cAMP ). P i = inorga nic phos pha te. Glucos e ca nnot diffus e dire ctly into ce lls but e nte rs by one of two tra ns - port me cha nis ms : a Na +-inde pe nde nt, fa cilita te d diffus ion tra ns port s ys - te m or a n ATP-de pe nde nt Na +-monos a ccha ride cotra ns port s ys te m. Ae ro bic Anae ro bic A 6-P Glucona te Ribulos e 5-P 6-P Gluconola ctone Glycoge n UDP -Glucos e Ga la ctos e Ga la ctos e 1-P B g lyc o lys is C g lyc o lys is Ribos e 5-P Glucos e 1-P UDP -Ga la ctos e Glucos e 6-P Glucos e Gluc o s e 6-P Gluc o s e Gluc o s e 6-P Gluc o s e Xylulos e 5-P S e dohe ptulos e 7-P Fructos e 6-P Fructos e Erythros e 4-P Fructos e 1,6-bis -P Fruc to s e 6-P Fruc to s e 6-P Glyce ra lde hyde Fructos e 1-P Glyce ra lde hyde 3-P Glyce ra lde hyde 3-P Dihydroxya ce tone -P Fruc to s e 1,6-bis pho s phate Fruc to s e 1,6-bis pho s phate 1,3-Bis phos phoglyce ra te Glyce rol-P Glyce rol 3-P hos phoglyce ra te Tria cylglyce rol Glyce ralde hyde 3-P Dihydro xy- Glyc e ralde hyde 3-P Dihydro xy- 2-P hos phoglyce ra te + + Ala Fa tty a cyl CoA Fa tty a cid NAD ac e to ne -P NAD ac e to ne -P Cys Gly Ser P hos phoe nolpyruva te NADH NADH Thr La cta te P yruva te Try CO 2 CO 2 Ma lonyl CoA Le u 1,3-Bis pho s pho g lyc e rate 1,3-Bis pho s pho g lyc e rate P he NH3 CO 2 Tyr Ace tyl-CoA Ace toa ce ta te Trp As n Ca rba moyl-P β-Hydroxybutyra te Lys 3-Pho s pho g lyc e rate 3-Pho s pho g lyc e rate As pa rta te Oxa loa ce ta te Citra te Citrulline Ma la te Is ocitra te Gln Argininos uccina te CO 2 P ro 2-Pho s pho g lyc e rate 2-Pho s pho g lyc e rate Ornithine Fuma ra te α-Ke togluta ra te Glu His CO 2 Arg S uccina te S uccinyl CoA Me thylma lonyl CoA Ure a Arginine Pho s pho e no lpyruvate Pho s pho e no lpyruvate Ile P ropionyl-CoA Me t P he Va l Ace tyl CoA Oxida tive Tyr Thr Fa tty a cyl-CoA phos phoryla tion Pyruvate Lac tate Pyruvate (odd-numbe r ca rbons ) Fig ure 8.9 A. Glycolys is s hown a s one of the e s s e ntia l pa thwa ys of e ne rgy me ta bolis m. B. Re a ctions of a e robic glycolys is. C. Re a ctions of a na e robic glycolys is. NAD(H) = nicotina mide a de nine dinucle otide ; P = phos pha te. t ahir99-VRG & vip.p ersianss.ir V. Re a ctions of Glycolys is 97 A. S o dium-inde pe nde nt fac ilitate d diffus io n trans po rt s ys te m Gluc o s e This s ys te m is me dia te d by a fa mily of 14 glucos e tra ns porte rs Gluc o s e Extra ce llula r trans po rte r found in ce ll me mbra ne s. The y a re de s igna te d GLUT-1 to GLUT- s pa ce (s tate 1) 14 (glucos e tra ns porte r is oforms 1–14). The s e monome ric prote in Ce ll me mbra ne tra ns porte rs e xis t in the me mbra ne in two conforma tiona l s ta te s GLUT (Figure 8.10). Extra ce llula r glucos e binds to the tra ns porte r, which Cytos ol the n a lte rs its conforma tion, tra ns porting glucos e a cros s the ce ll me mbra ne. Gluc o s e 1. Tis s ue s pe c ific ity o f g luc o s e trans po rte r g e ne e xpre s s io n: The Extra ce llula r trans po rte r GLUTs dis pla y a tis s ue -s pe cific pa tte rn of e xpre s s ion. For e xa m- s pa ce (s tate 2) ple , GLUT-3 is the prima ry glucos e tra ns porte r in ne urons. GLUT-1 is a bunda nt in e rythrocyte s a nd the blood–bra in ba rrie r but is low in Cytos ol a dult mus cle , whe re a s GLUT-4 is a bunda nt in mus cle a nd a dipos e tis s ue. [Note : The numbe r of GLUT-4 tra ns porte rs a ctive in the s e tis s ue s is incre a s e d by ins ulin. (Se e p. 311 for a dis cus s ion of ins u- lin a nd glucos e tra ns port.)] GLUT-2 is a bunda nt in live r, kidne y, a nd β ce lls of the pa ncre a s. The othe r GLUT is oforms a ls o ha ve Fig ure 8.10 tis s ue -s pe cific dis tributions. S che ma tic re pre s e nta tion of the fa cilita te d tra ns port of glucos e 2. S pe c ialize d func tio ns o f g luc o s e trans po rte r is o fo rms : In fa cili- through a ce ll me mbra ne. [Note : ta te d diffus ion, tra ns porte r-me dia te d glucos e move me nt is down Glucos e tra ns porte r prote ins a re monome ric a nd conta in 12 a conce ntra tion gra die nt (tha t is , from a high glucos e conce ntra - tra ns me mbra ne α he lice s.] tion to a lowe r one a nd, the re fore , doe s not re quire e ne rgy). For e xa mple , GLUT-1, GLUT-3, a nd GLUT-4 a re prima rily involve d in glucos e upta ke from the blood. In contra s t, GLUT-2, in the live r a nd kidne y, ca n e ithe r tra ns port glucos e into the s e ce lls whe n blood glucos e le ve ls a re high or tra ns port glucos e from the s e cells whe n blood glucos e le ve ls a re low (for e xa mple , during fa s ting). GLUT-5 is unus ua l in tha t it is the prima ry tra ns porte r for fructos e (not glu- Gluc o s e cos e ) in the s ma ll inte s tine a nd the te s te s. B. S o dium–mo no s ac c haride c o trans po rt s ys te m Ene rg y- 2ADP 2ATP inve s tme nt This is a n e ne rgy-re quiring proce s s tha t tra ns ports glucos e “a ga ins t” phas e a conce ntra tion gra die nt (tha t is , from low glucos e conce ntra tions outs ide the ce ll to highe r conce ntra tions within the ce ll). This s ys te m is a tra ns porte r-me dia te d proce s s in which the move me nt of glucos e is couple d to the conce ntra tion gra die nt of Na +, which is tra ns porte d Ene rg y- 4ADP 4ATP g e ne ratio n into the ce ll a t the s a me time. The tra ns porte r is a s odium-de pe nde nt phas e glucos e tra ns porte r (S GLT). This type of tra ns port occurs in the e pi- the lia l ce lls of the inte s tine (s e e p. 87), re na l tubule s , a nd choroid 2NAD+ 2NADH ple xus. [Note : The choroid ple xus , pa rt of the blood–bra in ba rrie r, a ls o conta ins GLUT-1.] 2Pyruvate V. REACTIONS OF GLYCOLYS IS Ne t (ae ro bic g lyc o lys is ): Gluc o s e 2Pyruvate The conve rs ion of glucos e to pyruva te occurs in two s ta ge s (Figure 2ADP 2ATP 2NAD+ 2NADH 8.11). The firs t five re a ctions of glycolys is corre s pond to a n e ne rgy– inve s tme nt pha s e in which the phos phoryla te d forms of inte rme dia te s a re s ynthe s ize d a t the e xpe ns e of ATP. The s ubs e que nt re a ctions of Fig ure 8.11 glycolys is cons titute a n e ne rgy–ge ne ra tion pha s e in which a ne t of two Two pha s e s of a e robic glycolys is. mole cule s of ATP a re forme d by s ubs tra te -le ve l phos phoryla tion (s e e NAD(H) = nicotina mide a de nine p. 102) pe r glucos e mole cule me ta bolize d. dinucle otide. t ahir99-VRG & vip.p ersianss.ir 98 8. Introduction to Me ta bolis m a nd Glycolys is A. Pho s pho rylatio n o f g luc o s e O C H Phos phoryla te d s uga r mole cule s do not re a dily pe ne tra te ce ll me m- H C OH HO C H bra nes because the re are no spe cific transmembrane carriers for the se H C OH compounds a nd be ca us e the y a re too pola r to diffus e through the H C OH lipid core of me mbra ne s. The irre versible phosphoryla tion of glucose H C OH (Figure 8.12), therefore, effective ly traps the suga r as cytosolic glucose H 6-phos pha te , the re by committing it to furthe r me ta bolis m in the ce ll. D-Gluc o s e ATP Ma mma ls ha ve four (I–IV) is ozyme s of the e nzyme he xokina s e tha t He xokina s e cata lyze the phosphorylation of glucose to glucose 6-phosphate. Glucokina s e ADP 1. He xo kinas e s I–III: In mos t tis s ue s , phos phoryla tion of glucos e is O ca ta lyze d by one of the s e is ozyme s of he xokina s e , which is one C H of thre e re gula tory e nzyme s of glycolys is (s e e a ls o phos phofruc- H C OH tokina s e a nd pyruva te kina s e ). The s e is ozyme s ha ve broa d s ub- HO C H H C OH s tra te s pe cificity a nd a re a ble to phos phoryla te s e ve ra l he xos e s H C OH in a ddition to glucose. The y a re inhibite d by the rea ction product, H C O P glucose 6-phosphate, which accumulates when further metabolism H of this hexose phosphate is re duced. Hexokinases I-III have a low Gluc o s e 6-pho s phate Michaelis consta nt (Km ) (a nd, therefore , a high affinity; see p. 59) for glucose. This permits the efficient phosphorylation a nd subsequent Fig ure 8.12 metabolism of glucose eve n when tissue concentrations of glucose Ene rgy-inve s tme nt pha s e : a re low (Figure 8.13). These isozymes, howeve r, ha ve a low maxi- phos phoryla tion of glucos e. [Note : ma l ve locity ([Vma x] s e e p. 59) for glucos e a nd, the re fore , do not Kina s e s utilize ATP comple xe d s e que s te r (tra p) ce llula r phos pha te in the form of phos phoryla te d with a diva le nt me ta l ion, mos t typica lly Mg 2+.] hexoses, or phosphorylate more sugars tha n the cell can use. 2. He xo kinas e IV (o r, g luc o kinas e ): In live r pa re nchyma l ce lls a nd β ce lls of the pa ncre a s , glucokina s e (the he xokina s e IV is ozyme ) is the pre domina nt e nzyme re s pons ible for the phos phoryla tion of glucos e. In β ce lls , glucokina s e functions a s a glucos e s e ns or, de te rmining the thre s hold for ins ulin s e cre tion (s e e p. 309). [Note : Co nc e ntratio n He xokina s e IV a ls o s e rve s a s a glucos e s e ns or in ne urons of o f fas ting blo o d g luc o s e the hypotha la mus , pla ying a ke y role in the a dre ne rgic re s pons e Vma x Glucokina s e to hypoglyce mia (s e e p. 315.] In the live r, the e nzyme fa cilita te s glucos e phos phoryla tion during hype rglyce mia. De s pite the popu- la r but mis le a ding na me glucokina s e , the s uga r s pe cificity of the e nzyme is s imila r to tha t of othe r he xokina s e is ozyme s. Enzyme ac tivity Glu c o kin a s e a. Kine tic s : Glucokina s e diffe rs from he xokina s e s I–III in s e ve ra l importa nt prope rtie s. For e xa mple , it ha s a much highe r Km , Vma x re quiring a highe r glucos e conce ntra tion for ha lf-s a tura tion He xokina s e (s e e Figure 8.13). Thus , glucokina s e functions only whe n He xo kin a s e the intra ce llula r conce ntra tion of glucos e in the he pa tocyte is 0 e le va te d s uch a s during the brie f pe riod following cons umption 0 5 10 15 20 of a ca rbohydra te -rich me a l, whe n high le ve ls of glucos e a re Km Km de live re d to the live r via the porta l ve in. Glucokina s e ha s a high He xokina s e Glucokina s e Vma x, a llowing the live r to e ffe ctive ly re move the flood of glucos e Gluc o s e c o nc e ntratio n (mmo l/l) de live re d by the porta l blood. This pre ve nts la rge a mounts of glucos e from e nte ring the s ys te mic circula tion following s uch a Fig ure 8.13 me a l the re by minimizing hype rglyce mia during the a bs orptive Effe ct of glucos e conce ntra tion on pe riod. [Note : GLUT-2 ins ure s tha t blood glucos e e quilibra te s the ra te of phos phoryla tion ra pidly a cros s the me mbra ne of the he pa tocyte.] ca ta lyze d by he xokina s e a nd glucokina s e. Km = Micha e lis b. Regulation by fructos e 6-phos phate and glucos e: cons ta nt; Vma x = ma xima l ve locity. Glucokinase activity is not directly inhibite d by glucose 6-phos- phate as are the other hexokinase s but, rather, is indire ctly inhib- ited by fructose 6-phosphate (which is in equilibrium with glucose t ahir99-VRG & vip.p ersianss.ir V. Re a ctions of Glycolys is 99 6-phospha te , a product of glucokina se ) and is indire ctly s timu- Gluc o s e la te d by glucos e (a s ubs tra te of glucokina s e ) via the following mecha nism. Glucokina se re gula tory protein (GKRP) in the liver GLUT-2 PLAS MA MEMBRANE regulates the activity of glucokinase through reversible binding. CYTOS OL In the presence of fructose 6-phosphate, glucokinase is translo- Gluc o s e ca ted into the nucleus and binds tightly to the re gula tory protein, GK thereby rendering the enzyme ina ctive (Figure 8.14). When glu- Gluc o s e + 6-pho s phate NUCLEUS cose levels in the blood (a nd also in the hepa tocyte, as a result + of GLUT-2) increase, glucokinase is relea sed from the regulatory GK GKRP protein, and the enzyme reenters the cytosol whe re it phosphor- Fruc to s e 6-pho s phate ylates glucose to glucose 6-phosphate. [Note : Fructose 1-phos- phate inhibits forma tion of the glucokina se–GKRP complex.] Gluc o kinas e re g ulato ry Glu c o - pro te in Pyruvate kin a s e (GK) (GKRP) (inac tive ) Glucokina s e functions a s a glucos e s e ns or in the ma inte na nce of blood glucos e home os ta s is. Ina ctiva ting muta tions of glucokina s e a re the Fig ure 8.14 ca us e of a ra re form of dia be te s , ma turity ons e t Re gula tion of glucokina s e a ctivity by glucokina s e re gula tory prote in. dia be te s of the young type 2 (MODY 2) tha t is GLUT = glucos e tra ns porte r. cha ra cte rize d by impa ire d ins ulin s e cre tion. B. Is o me rizatio n o f g luc o s e 6-pho s phate O The is ome riza tion of glucos e 6-phos pha te to fructos e 6-phos pha te is C H ca ta lyze d by phos phoglucos e is ome ra s e (Figure 8.15). The re a ction H C OH HO C H is re a dily re ve rs ible a nd is not a ra te -limiting or re gula te d s te p. H C OH H C OH C. Pho s pho rylatio n o f fruc to s e 6-pho s phate H C O P H The irre ve rs ible phos phoryla tion re a ction ca ta lyze d by phos pho- Gluc o s e 6-pho s phate (a ldos e ) fructokina s e -1 (P FK-1) is the mos t importa nt control point a nd the ra te -limiting a nd committe d s te p of glycolys is (Figure 8.16). P FK-1 is P hos phoglucos e is ome ra s e controlle d by the a va ila ble conce ntra tions of the s ubs tra te s ATP a nd fructos e 6-phos pha te a s we ll a s by re gula tory s ubs ta nce s de s cribe d H be low. H C OH C O 1. Re g ulatio n by e ne rg y le ve ls within the c e ll: P FK-1 is inhibite d HO C H a llos te rica lly by e le va te d le ve ls of ATP , which a ct a s a n “e ne rgy- H C OH rich” s igna l indica ting a n a bunda nce of high-e ne rgy compounds. H C OH H C O P Ele va te d le ve ls of citra te , a n inte rme dia te in the TCA cycle (s e e H p. 109), a ls o inhibit P FK-1. [Note : Inhibition by citra te fa vors the Fruc to s e 6-pho s phate (ke tos e ) us e of glucos e for glycoge n s ynthe s is (s e e p. 125).] Conve rs e ly, P FK-1 is a ctiva te d a llos te rica lly by high conce ntra tions of AMP , which s igna l tha t the ce ll’s e ne rgy s tore s a re de ple te d. Fig ure 8.15 Aldos e -ke tos e is ome riza tion of glucos e 6-phos pha te to fructos e 2. Re g ulatio n by fruc to s e 2,6-bis pho s phate : Fructos e 2,6-bis phos - 6-phos pha te. P = phos pha te. pha te is the mos t pote nt a ctiva tor of P FK-1 (s e e Figure 8.16) a nd is a ble to a ctiva te the e nzyme e ve n whe n ATP le ve ls a re high. Fructos e 2,6-bis phos pha te is forme d from fructos e 6-phos pha te by phos phofructokina s e -2 (P FK-2), a n e nzyme diffe re nt tha n P FK-1. P FK-2 is a bifunctiona l prote in tha t ha s both the kina s e a ctivity tha t produce s fructos e 2,6-bis phos pha te a nd the phos - pha ta s e a ctivity tha t de phos phoryla te s fructos e 2,6-bis phos pha te ba ck to fructos e 6-phos pha te. In the live r, the kina s e doma in is a ctive if de phos phoryla te d a nd is ina ctive if phos phoryla te d (Figure 8.17). [Note : Fructos e 2,6-bis phos pha te is a n inhibitor t ahir99-VRG & vip.p ersianss.ir 100 8. Introduction to Me ta bolis m a nd Glycolys is Fruc to s e 6-pho s phate of fructos e 1,6-bis phos pha ta s e , a n e nzyme of glucone oge ne s is (s e e p. 120). The re ciproca l a ctions of fructos e 2,6-bis phos pha te ATP ATP, c itrate on glycolys is (a ctiva tion) a nd glucone oge ne s is (inhibition) e ns ure AMP tha t both pa thwa ys a re not fully a ctive a t the s a me time , pre ve nt- + + P hos phofructo- kina s e -1 Fruc to s e 2,6-bis - ing a futile cycle in which glucos e would be conve rte d to pyruva te ADP pho s phate followe d by re s ynthe s is of glucos e from pyruva te.] H H C O P C O a. During the we ll-fe d s tate : De cre a s e d le ve ls of gluca gon a nd HO C H e le va te d le ve ls of ins ulin, s uch a s occur following a ca rbohy- H C OH dra te -rich me a l, ca us e a n incre a s e in fructos e 2,6-bis phos - H C OH pha te a nd, thus , in the ra te of glycolys is in the live r (s e e Figure H C O P H 8.17). Fructos e 2,6-bis phos pha te , the re fore , a cts a s a n intra - Fruc to s e 1,6-bis pho s phate ce llula r s igna l, indica ting tha t glucos e is a bunda nt. O Aldola s e H C H H C O P b. During fas ting : Ele va te d le ve ls of gluca gon a nd low le ve ls of H C OH C O ins ulin, s uch a s occur during fa s ting (s e e p. 327), de cre a s e H C O P Trios e H C OH the intra ce llula r conce ntra tion of he pa tic fructos e 2,6-bis pho- H phos pha te is omera s e H s pha te. This re s ults in inhibition of glycolys is a nd a ctiva tion of glucone oge ne s is. Glyc e ralde hyde Dihydro xyac e to ne 3-pho s phate pho s phate D. Cle avag e o f fruc to s e 1,6-bis pho s phate Fig ure 8.16 Aldola s e cle a ve s fructos e 1,6-bis phos pha te to dihydroxya ce tone Ene rgy–inve s tme nt pha s e (continue d): phos pha te a nd glyce ra lde hyde 3-phos pha te (s e e Figure 8.16). The Conve rs ion of fructos e 6-phos pha te re a ction is re ve rs ible a nd not re gula te d. [Note : Aldola s e B, the is o- to trios e phos pha te s. P = phos pha te ; AMP = a de nos ine monophos pha te. form found prima rily in the live r, a ls o cle a ve s fructos e 1-phos pha te a nd functions in the me ta bolis m of die ta ry fructos e (s e e p. 138).] Ins ulin Gluc ag o n (hig h) (lo w) Re ce ptor Re ce ptor CELL MEMBRANE Ad e n ylyl CYTOS OL Ac tivatio n o f many e nzyme s c yc la s e Hig h ins ulin /g luc ag o n ratio c aus e s ATP c AMP 1 de c re as e d c AMP and re duc e d le ve ls o f ac tive p ro te in kin a s e A. Ac tive p ro te in kin a s e A De c re as e d p ro te in kin a s e A ac tivity G ly c o ly s is 2 favo rs de pho s pho rylatio n o f F r u c t o s e 6 -p h o s p h a t e Fruc to s e 6-pho s phate P FK-2/FBP -2. Glucos e 6-P Glucos e AT P P Fr u c t o s e 6 -P Bifunctiona l e nzyme ATP ADP P h o s p h o fru c to - + F r u c t o s e 1 ,6 -b is -P kin a s e -1 P FK-2 FBP -2 (a ctive ) (ina ctive ) PFK-2 FBP -2 Glyce ra lde hyde 3-P DHAP ADP (inactive) (a ctive ) 1,3-Bis phos phoglyce ra te 3-P hos phoglyce ra te P Bifunctiona l e nzyme F r u c t o s e 1 ,6 -b is p h o s p h a t e Fruc to s e 2,6-bis pho s phate 2-P hos phoglyce ra te P hos phoe nolpyruva te De pho s pho rylate d P FK-2 do main is ac tive , Ele vate d c o nc e ntratio n o f fruc to s e 2,6-bis pho s phate ac tivate s 3 whe re as FBP -2 is inac tive whic h favo rs 4 La cta te P yruva te P FK-1, whic h le ads to an inc re as e d rate o f g lyc o lys is. fo rmatio n o f fruc to s e 2,6-bis pho s phate. Fig ure 8.17 Effe ct of e le va te d ins ulin conce ntra tion on the intra ce llula r conce ntra tion of fructos e 2,6-bis phos pha te in live r. P FK-2 = phos phofructokina s e -2; FBP -2 = fructos e 2,6-bis phos pha ta s e ; cAMP = cyclic AMP ; P = phos pha te. t ahir99-VRG & vip.p ersianss.ir V. Re a ctions of Glycolys is 101 E. Is o me rizatio n o f dihydro xyac e to ne pho s phate O Trios e phos pha te is ome ra s e inte rconve rts dihydroxya ce tone phos - C H H C OH pha te (DHAP ) a nd glyce ra lde hyde 3-phos pha te (s e e Figure 8.16). H C O P DHAP mus t be is ome rize d to glyce ra lde hyde 3-phos pha te for furthe r H me ta bolis m by the glycolytic pa thwa y. This is ome riza tion re s ults in Glyc e ralde hyde 3-pho s phate the ne t production of two mole cule s of glyce ra lde hyde 3-phos pha te Pi from the cle a va ge products of fructos e 1,6-bis phos pha te. [Note : DHAP is utilize d in tria cylglyce rol s ynthe s is (s e e p. 188).] Glyce ra lde hyde NAD+ 3-phos pha te de hydroge na s e F. Oxidatio n o f g lyc e ralde hyde 3-pho s phate NADH + H+ The conve rs ion of glyce ra lde hyde 3-phos pha te to 1,3-bis phos pho- O C O~ P glyce ra te (1,3-BP G) by glyce ra lde hyde 3-phos pha te de hydroge na s e H C OH is the firs t oxida tion-re duction re a ction of glycolys is (Figure 8.18). H C O P H [Note : Be ca us e the re is only a limite d a mount of NAD+ in the ce ll, the NADH forme d by this re a ction mus t be re oxidize d to NAD+ for 1,3-Bis pho s pho g lyc e rate Muta s e O glycolys is to continue. Two ma jor me cha nis ms for oxidizing NADH ADP C O- a re 1) the NADH-linke d conve rs ion of pyruva te to la cta te (a na e robic; H C O P P hos pho- s e e p. 96) a nd 2) oxida tion of NADH via the re s pira tory cha in (a e ro- glyce ra te H C O P bic; s e e p. 74). The la tte r re quire s the ma la te -a s pa rta te a nd glyce rol kina s e H ATP 2,3-Bis pho s pho - 3-phos pha te s ubs tra te s huttle s (s e e p. 79.] g lyc e rate O H2 O 1. Synthe s is o f 1,3-bis pho s pho g lyc e rate : The oxida tion of the a lde - C O- H C OH P hos pha ta s e hyde group of glyce ra lde hyde 3-phos pha te to a ca rboxyl group is H C O P couple d to the a tta chme nt of P i to the ca rboxyl group. The high- H Pi e ne rgy phos pha te group a t ca rbon 1 of 1,3-BPG cons e rve s much 3-Pho s pho g lyc e rate of the fre e e ne rgy produce d by the oxida tion of glyce ra lde hyde P hos pho- 3-phos pha te. The e ne rgy of this high-e ne rgy phos pha te drive s the glyce ra te muta s e s ynthe s is of ATP in the ne xt re a ction of glycolys is. O C O- 2. Me c hanis m o f ars e nic po is o ning : The toxicity of a rs e nic is H C O P due prima rily to the inhibition by triva le nt a rs e nic (a rs e nite ) of H C OH e nzyme s s uch a s the pyruva te de hydroge na s e comple x, which H 2-Pho s pho g lyc e rate re quire lipoic a cid a s a coe nzyme (s e e p. 110). Howe ve r, pe n- ta va le nt a rs e nic (a rs e na te ) ca n pre ve nt ne t ATP a nd NADH Enola s e production by glycolys is without inhibiting the pa thwa y its e lf. It H2 O doe s s o by compe ting with P i a s a s ubs tra te for glyce ra lde hyde O 3-phos pha te de hydroge na s e , forming a comple x tha t s ponta ne - C O- ous ly hydrolyze s to form 3-phos phoglyce ra te (s e e Figure 8.18). C O~ P H C H By bypa s s ing the s ynthe s is of a nd phos pha te tra ns fe r from 1,3- Pho s pho e no lpyruvate BP G, the ce ll is de prive d of e ne rgy us ua lly obta ine d from the glycolytic pa thwa y. [Note : Ars e na te a ls o compe te s with P i on the ADP P yruva te Fruc to s e 1,6- F 1 doma in of ATP s yntha s e (s e e p. 77), re s ulting in forma tion of + kina s e bis pho s phate ADP -a rs e na te tha t is ra pidly hydrolyze d.] ATP O C O- 3. S ynthe s is o f 2,3-bis pho s pho g lyc e rate in re d blo o d c e lls : C O S ome of the 1,3-BP G is conve rte d to 2,3-BP G by the a ction of H C H bis phos phoglyce ra te muta s e (s e e Figure 8.18). 2,3-BP G, which H Pyruvate is found in only tra ce a mounts in mos t ce lls , is pre s e nt a t high conce ntra tion in re d blood ce lls (RBCs ) a nd s e rve s to incre a s e O 2 de live ry (s e e p. 31). 2,3-BP G is hydrolyze d by a phos pha ta s e Fig ure 8.18 to 3-phos phoglyce ra te , which is a ls o a n inte rme dia te in glycolys is Ene rgy–ge ne ra ting pha s e : (s e e Figure 8.18). In the RBC, glycolys is is modifie d by inclus ion conve rs ion of glyce ra lde hyde 3-phos pha te to pyruva te. NAD(H) = of the s e “s hunt” re a ctions. nicotina mide a de nine dinucle otide P = phos pha te ; P i = inorga nic phos pha te. 102 8. Introduction to Me ta bolis m a nd Glycolys is G. S ynthe s is o f 3-pho s pho g lyc e rate , pro duc ing ATP Whe n 1,3-BP G is conve rte d to 3-phos phoglyce ra te , the high-e ne rgy phos pha te group of 1,3-BP G is us e d to s ynthe s ize ATP from ADP (s e e Figure 8.18). This re a ction is ca ta lyze d by phos phoglyce ra te kina s e , which, unlike mos t othe r kina s e s , is phys iologica lly re ve rs - ible. Be ca us e two mole cule s of 1,3-BPG a re forme d from e a ch glu- cos e mole cule , this kina s e re a ction re pla ce s the two ATP mole cule s cons ume d by the e a rlie r forma tion of glucos e 6-phos pha te a nd fruc- tos e 1,6-bis phos pha te. [Note : This is a n e xa mple of s ubs tra te -le ve l phos phoryla tion, in which the e ne rgy ne e de d for the production of a high-e ne rgy phos pha te come s from a s ubs tra te ra the r tha n from the e le ctron tra ns port cha in (s e e J. be low a nd p. 113 for othe r e xa mple s ).] H. Shift of the phos phate group The s hift of the phos pha te group from ca rbon 3 to ca rbon 2 of phos - phoglyce ra te by phos phoglyce ra te muta s e is fre e ly re ve rs ible (s e e Figure 8.18). I. De hydratio n o f 2-pho s pho g lyc e rate The de hydra tion of 2-phos phoglyce ra te by e nola s e re dis tribute s the energy within the substrate, resulting in the formation of phosphoenol- pyruva te (P EP ), which conta ins a high-e ne rgy e nol phos pha te (s e e Figure 8.18). The reaction is reversible despite the high-energy nature of the product. [Note: Fluoride inhibits enolase, and water fluoridation reduces lactate production by mouth bacteria, decreasing dental caries.] J. Fo rmatio n o f pyruvate , pro duc ing ATP The conve rs ion of P EP to pyruva te is ca ta lyze d by pyruva te kina s e Gluc ag o n (P K), the third irre ve rs ible re a ction of glycolys is. The high-e ne rgy e nol phos pha te in PEP is us e d to s ynthe s ize ATP from ADP a nd is a nothe r Re ce ptor e xa mple of s ubs tra te -le ve l phos phoryla tion (s e e Figure 8.18). Ade nylyl cycla s e 1. Fe e dfo rward re g ulatio n: P K is a ctiva te d by fructos e 1,6-bis pho- ATP c AMP + PP i s pha te , the product of the phos phofructokina s e -1 re a ction. This fe e dforwa rd (ins te a d of the more us ua l fe e dba ck) re gula tion ha s the e ffe ct of linking the two kina s e a ctivitie s : incre a s e d phos - Ac tive p ro te in kin a s e A phofructokina s e a ctivity re s ults in e le va te d le ve ls of fructos e PEP 1,6-bis phos pha te , which a ctiva te s P K. ADP 2. Co vale nt mo dulatio n o f pyruvate kinas e : P hos phoryla tion by P a cAMP -de pe nde nt prote in kina s e le a ds to ina ctiva tion of the ATP ADP he pa tic is ozyme of P K (Figure 8.19). Whe n blood glucos e le ve ls P yruva te P yruva te kina s e kina s e a re low, e le va te d gluca gon incre a s e s the intra ce llula r le ve l of (a ctive ) (ina ctive ) cAMP , which ca us e s the phos phoryla tion a nd ina ctiva tion of P K in the live r only. The re fore , P EP is una ble to continue in glycoly- ATP s is a nd, ins te a d, e nte rs the glucone oge ne s is pa thwa y. This , in Pyruvate pa rt, e xpla ins the obs e rve d inhibition of he pa tic glycolys is a nd s timula tion of glucone oge ne s is by gluca gon. De phos phoryla tion Fig ure 8.19 of P K by a phos pha ta s e re s ults in re a ctiva tion of the e nzyme. Cova le nt modifica tion of he pa tic pyruvate kinase results in inactivation 3. Pyruvate kinas e de fic ie nc y: Ma ture RBCs la ck mitochondria a nd of the e nzyme. cAMP = cyclic AMP ; a re, therefore , completely dependent on glycolysis for ATP produc- P EP = phos phoe nolpyruva te ; tion. ATP is required to meet the metabolic needs of RBCs and to P = phos pha te ; P P i = pyrophos pha te. fue l the ion pumps nece ss a ry for the ma intena nce of the fle xible , biconca ve sha pe that a llows them to squeeze through narrow capil- V. Re a ctions of Glycolys is 103 laries. The anemia observed in glycolytic enzyme deficiencie s is a Glucos e 6-P Glucos e conseque nce of the reduce d rate of glycolysis, lea ding to decrease d Theee6-P nzyme may s ho w ATP production. The re s ulting a lte ra tions in the RBC me mbra ne Fructos an abno rmal re s po ns e to le a d to cha nge s in ce ll s ha pe a nd, ultima te ly, to pha gocytos is by the ac tivato r fruc to s e uctos e 1,6-bis -P Fructos ce lls of the reticuloendothelial system, pa rticularly macropha ges of 1,6-bis pho s phate. the splee n. The premature dea th and lysis of RBCs result in hemo- Glyc ce ra lde hyde 3-P