Principles of Biochemistry Lecture 17 PDF - Spring 2024 - Weill Cornell Medicine-Qatar

Principles of Biochemistry SPRING 2024 Professor: Moncef LADJIMI [email protected] Office: C-169 As faculty of Weill Cornell Medical College in Qatar we are committed to providing transparency for any and all external relationships prior to giving an academic presentation. I, Moncef LADJ...

Principles of Biochemistry SPRING 2024 Professor: Moncef LADJIMI [email protected] Office: C-169 As faculty of Weill Cornell Medical College in Qatar we are committed to providing transparency for any and all external relationships prior to giving an academic presentation. I, Moncef LADJIMI DO NOT have a financial interest in commercial products or services. Lecture 17 The Pentose Phosphate Pathway (PPP) Also called: phosphogluconate pathway or hexose monophosphate shunt Additional material for this lecture may be found in: § Lehninger’s Biochemistry (8th ed), chapter 14: p. 546-552 PENTOSE PHOSPHATE PATHWAY OF GLUCOSE OXYDATION The PPP (also called phosphogluconate pathway or hexose monophosphate shunt), is not really to provide another pathway for glucose oxidation nor is it for ATP production. Rather, the PPP caters for specialized metabolic needs à making other products… Physiological role of Pentose phosphate pathway: - Synthesis of Ribose 5-phosphate, for RNA, DNA and coenzyme synthesis of NAD, NADP, FAD, CoA, in rapidly dividing cells (bone marrow, skin, intestine mucosa, tumors…) - Formation of NADPH – “reducing power” - for reductive biosynthetic processes, such as the lipids synthesis (in adipose, lactating mammary gland, liver) and cholesterol and steroid hormones synthesis (adrenals, gonads), and - for maintaining a high reduction potential in cells exposed to direct oxygen, to prevent oxidative damage from oxygen radicals (erythrocytes, cells of lens and cornea exposed to O2). THE PENTOSE PHOSPHATE PATHWAY HAS TWO PHASES 1/ Oxydative phase à 2 products: u NADPH formed in the oxidative phase is used: - to support reductive biosynthesis - and to reduce glutathione,GSSG, to GSH u The other product of the oxidative phase is ribose 5-phosphate, - which serves as a precursor for nucleotides (nucleic acids) and coenzymes. 2/ Non-oxydative phase: Recycling - In cells that are not using ribose 5-phosphate for biosynthesis, the non-oxidative phase recycles six molecules of pentose phosphate into five molecules of the hexose phosphate (in part through gluconeogenesis). - This phase provides a route for excess pentose sugars in the diet to the mainstream of glucose metabolism THE OXYDATIVE PHASE OF THE PENTOSE PHOSPHATE PATHWAY PRODUCES PENTOSE PHOSPHATES AND NADPH Rate limiting step In some tissues (rapidly dividing cells: bone marrow, skin, intestine mucosa, tumors etc.) that require ribose-5phosphate , the pentose phosphate pathway ends at this point with the production of - 1 CO2, - 2 NADPH - 1 ribose 5-phosphate - - The 3 first reactions (including G6PDH) are irreversible… Ribose 5-phosphate is used for nucleotide (DNA, RNA) and coenzymes ( NADH, FADH2, CoA…) synthesis. NADPH is used for biosynthetic reductions (fatty acid synthesis in adipose and in lactating mammary gland…) THE NON-OXYDATIVE PHASE OF THE PENTOSE PHOSPHATE PATHWAY RECYCLES PENTOSE PHOSPHATES TO GLUCOSE PHOSPHATE (RIBOSE 5-P TO GLUCOSE 6-P) In other tissues that require primarily NADPH for reductive biosynthesis (fatty acid synthesis in adipose and lactating mammary glands), and active cholesterol and steroids synthesis (in liver, adrenals, gonads…), or to counter the damaging effects of oxygen radicals (erythrocytes, cell of the lens, cornea…), the pentose phosphates produced by the oxidative phase are recycled into glucose 6-phosphate. In this non-oxidative phase, ribulose 5-phosphate is first epimerized into xylulose 5-phosphate (a). Ribulose 5-P Then, in a series of rearrangements, 6 five-carbon sugars are converted into 5 six-carbon sugars (b) Two enzymes are critical in the interconversion of sugars in this process: Transketolase and Transaldolase All reactions are reversible but shown here as unidirectional arrows to highlight the direction of reactions under continuous oxidation of G6P THE FIRST REACTION CATALYZED BY TRANSKETOLASE IN THE NONOXYDATIVE PHASE OF THE PENTOSE PHOSPHATE PATHWAY Transketolase catalyzes the transfer of a two-carbon fragment, carried temporarily on enzyme-bound TPP, from a ketose donor to an aldose acceptor. Both xylulose 5-phosphate and ribose 5-phosphate are available from the isomerization of ribulose 5-phosphate. This reaction leads to the conversion of two pentose phosphates to a triose phosphate and a sevencarbon sugar phosphate: The transfer of two carbons from xylulose 5-phosphate to ribose 5-phosphate produces glyceraldehyde 3-phosphate and sedoheptulose 7phosphate. THE REACTION CATALYZED BY TRANSALDOLASE IN THE NONOXYDATIVE PHASE OF THE PENTOSE PHOSPHATE PATHWAY Transaldolase is the second critical enzyme in the interconversions of the pentose phosphate pathway: A three-carbon fragment is removed from sedoheptulose 7phosphate and condensed with glyceraldehyde 3-phosphate forming fructose 6phosphate and erythrose 4-phosphate. THE SECOND REACTION CATALYZED BY TRANKETOLASE IN THE NONOXYDATIVE PHASE OF THE PENTOSE PHOSPHATE PATHWAY Transketolase is required a second time. Again, the donor xylulose 5-phosphate contributes a two-carbon fragment, but this time to erythrose 4-phosphate produced in the transaldolase reaction, to yield fructose 6-phosphate and glyceraldehyde 3-phosphate. Two iterations of this reaction (and previous reactions; see previous slides) will yield two molecules of glyceraldehyde 3-phosphate that can be converted to glucose by a reversal of the glycolytic pathway (gluconeogenesis reactions). Net effect: 6 Ribose-5-P à 5 Glucose-6-P + Pi NADPH REGULATES PARTITIONING BETWEEN GLYCOLYSIS AND THE PENTOSE PHOSPHATE PATHWAY The reactions of the glycolysis and the pentose phosphate pathways both occur in the cytosol. Which pathway glucose enters depends on the needs of the cell at that particular moment. These needs are reflected in the level of NADPH: If the level of NADPH is high, NADPH acts as an inhibitor of the first enzyme in the pathway, glucose 6-phosphate dehydrogenase G6PDHNADPH=Inhibitor (G6PDH). Glucose will then be available for + NADP =Activator glycolysis. If the level of NADPH is low (NADP+ is high), the pentose phosphate pathway will operate, through activation by NADP+. Thus, when NADPH is forming faster than it is being used for biosynthesis and glutathione reduction, [NADPH] rises and inhibits the first enzyme in the pentose phosphate pathway. As a result, more glucose 6-phosphate is available for glycolysis. JC08-ROLE OF NADPH AND GLUTATHIONE IN PROTECTING CELLS AGAINST HIGHLY REACTIVE OXYGEN DERIVATIVES Reduced glutathione (GSH, a tripeptide: g-glutamyl-cysteinylglycine) protects the cell by destroying hydrogen peroxide and hydroxyl free radicals (and becomes oxidized:GSSG). Non enzymatic “scavenging ” reaction Regeneration of GSH from its oxidized form (GSSG) requires the NADPH produced in the glucose 6phosphate dehydrogenase reaction. u G6PD deficiency Can be fatal – in cases of high oxidative stress – In presence of certain drugs, herbicides, and some foods fava beans=divicine) Gives resistance to malaria due to high oxidative stress in red blood cells that kills the parasite SUMMARY The oxidative phase: the main products are NADPH and ribose 5phosphate – NADPH is an electron donor Used in reductive biosynthesis of fatty acids and steroids Used in repair of oxidative damage – Ribose-5-phosphate is a biosynthetic precursor of nucleotides Used in DNA and RNA synthesis Used in synthesis of some coenzymes The non-oxidative phase recycles six molecules of the pentose phosphate into five molecules of the hexose phosphate Two enzymes are critical in the inter-conversion of sugars in the pentose phosphate pathway:Transketolase and Transaldolase RECAP Glycolysis, a process by which cells can extract a limited amount of energy from glucose under anaerobic conditions Gluconeogenesis, a process by which cells can use a variety of metabolites for the synthesis of glucose – The differences between glycolysis and gluconeogenesis How they are both made thermodynamically favorable How they are differentially regulated to avoid a futile cycle Pentose phosphate pathway, a process by which cells can generate reducing power (NADPH) that is needed for the biosynthesis of various compounds Remember to prepare for next lecture: Lehninger’s Biochemistry (8th ed), §chapter 15: p. 556-565