BCH3033 Biochemistry 1 Chapter 7A PDF

BCH3033: Biochemistry 1 Chapter 7A 02.27.2024 Donella Beckwith, Ph.D. [email protected] 1 Carbohydrates carbohydrates are aldehydes or ketones with at least two hydroxyl groups, or substances that yield such compounds on hydrolysis many carbohydrates have the empirical formula (CH2O)n aldehyde group...

BCH3033: Biochemistry 1 Chapter 7A 02.27.2024 Donella Beckwith, Ph.D. [email protected] 1 Carbohydrates carbohydrates are aldehydes or ketones with at least two hydroxyl groups, or substances that yield such compounds on hydrolysis many carbohydrates have the empirical formula (CH2O)n aldehyde group polyhydroxy groups ketone group 2 Classes of Carbohydrates monosaccharides = simple sugars, consist of a single polyhydroxy aldehyde or ketone unit oligosaccharides = short chains of monosaccharide units, or residues, joined by glycosidic bonds (< 12 monosaccharides) disaccharides = oligosaccharides with two monosaccharide units – example: sucrose (D-glucose and D-fructose) polysaccharides = sugar polymers with 10+ monosaccharide units – examples: cellulose (linear), glycogen (branched) 3 4 Question A(n) ________ is a(n) ________ with two monosaccharide units. A. B. C. D. oligosaccharide; polysaccharide polysaccharide; oligosaccharide disaccharide; oligosaccharide disaccharide; polysaccharide 5 The Two Families of Monosaccharides Are Aldoses and Ketoses aldose = carbonyl group is at an end of the carbon chain (in an aldehyde group) ketose = carbonyl group is at any other position (in a ketone group) 6 Question What chemical feature determines if a sugar is an aldose or a ketose? A. the direction of rotation of polarized light B. the position of the carbonyl carbon C. if the enantiomers resemble either glyceraldehyde or dihydroxyacetone D. if the epimers differ in configuration around one carbon or more than one carbon 7 Trioses trioses = simplest monosaccharides, three carbon backbone 8 Tetroses and Pentoses tetroses = four component of RNA component of DNA carbon backbone pentoses = five carbon backbone 9 Hexoses and Heptoses D-Glucose D-Fructose hexoses = six carbon backbone heptoses = seven carbon backbone 10 Question Glucose is a monosaccharide with a(n): A. B. C. D. aldehyde functional group and six carbons. ketone functional group and six carbons. aldehyde functional group and seven carbons. ketone functional group and seven carbons. 11 Monosaccharides Have Asymmetric Centers all monosaccharides (except dihydroxyacetone) contain 1 or more chiral carbon atoms – occur in optically active isomeric forms Enantiomers: two different optical isomers that are mirror images in general, a molecule with n chiral centers can have 2n stereoisomers Example: glucose has 4 chiral centers, 24 = 16 12 Fischer Projection Formulas used to represent three-dimensional sugar structures on paper bonds drawn horizontally indicate bonds that project out of the plane of the paper bonds drawn vertically project behind the plane of the paper 13 D Isomers and L Isomers reference carbon = chiral center most distant from the carbonyl carbon two groups of stereoisomers: – D isomers = configuration at reference carbon is the same as Dglyceraldehyde on the right (dextro) in a projection formula most hexoses of living organisms – L isomers = configuration at reference carbon is the same as Lglyceraldehyde on the left (levo) in a projection formula 14 Question Which statement is false regarding the enantiomers of glyceraldehyde? A. The enantiomers of glyceraldehyde are mirror images of each other. B. Because glyceraldehyde contains one chiral center, it has two enantiomers. C. In the Fischer projection formula for D-glyceraldehyde, the hydroxyl group is on the right. D. Glyceraldehyde does not occur in optically active isomeric forms. 15 D-Aldoses aldotriose aldotetrose Chiral carbons are red Chiral carbon most distant from the carbonyl carbon has the same 16 configuration as the chiral carbon in D-glyceraldehyde D-Ketoses 17 Numbering Carbons of a Sugar carbons are numbered beginning at the end of the chain near the carbonyl group D- “Chair” 18 Common Monosaccharides Have Cyclic Structures in aqueous solution, aldotetroses and all monosaccharides with 5+ backbone carbon atoms occur as cyclic structures – there is a covalent bond between the carbonyl group and the oxygen of a hydroxyl group “Chair” 19 Epimers epimers = two sugars that differ only in the configuration around one carbon atom 20 *Question A monosaccharide with a ketone functional group and seven carbons: A. B. C. D. can never exist in cyclic form. is a ketoheptose. is the monosaccharide found in RNA but not DNA. has a single chiral center. 21 Hemiacetals and Hemiketals hemiacetals or hemiketals = derivatives formed by a general reaction between alcohols and aldehydes or ketones – product of the first alcohol molecule addition – a five- or six-membered ring forms if the —OH and carbonyl groups are on the same molecule – Example: hemiacetal of D-glucose or hemiketal of D-fructose acetal or ketal = product of the second alcohol molecule addition – forms a glycosidic bond 22 Formation of Hemiacetals and Hemiketals 6 5 D-Glucose 1 α Intramolecular nucleophilic attack Carbonyl oxygen becomes an OH which points below (α) or above (β) the ring 23 Pyranoses and Furanoses pyranoses = six-membered ring compounds – form when the hydroxyl group at C-6 reacts with the keto group at C-2 – specific to fructose furanoses = fivemembered ring compounds – form when the hydroxyl group at C-5 reacts with the keto group at C-2 24 Pyranoses and Furanoses Fructose (aq) Pyranose fructose Furanose fructose 1 CH2OH 2 O HO 3 H H 4 OH H 5 OH 6 CH 2OH C5 –OH C2 - carbonyl 25 Pyranoses and Furanoses Fructose (aq) Pyranose fructose Furanose fructose 1 CH2OH H C6 –OH C2 - carbonyl 2 O HO 3 H H 4 OH H 5 OH 6 CH 2OH 26 Pyranoses and Furanoses fructose – can form 5 or 6 membered ring – 5 membered ring forms when the hydroxyl group at C-5 reacts with the keto group (because it is a ketone) at C-2 forming a hemiketalic bond b/w C2 and C5 **furanose form** – 6 membered ring forms when the hydroxyl group at C-6 reacts with the keto group at C-2 **pyranose form** 27 α and β Stereoisomeric Configurations reaction with the first alcohol molecule creates an additional chiral center (the carbonyl carbon) produces either of two stereoisomeric configurations: α and β anomers = isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom anomeric carbon = the carbonyl carbon atom “Chair” 28 Formation of the Two Cyclic Forms of D-Glucose reaction between the aldehyde group at C-1 and the hydroxyl group at C5 forms a hemiacetal linkage mutarotation = the interconversion of α and β anomers 29 Question What name is given to monosaccharides that differ in configuration about the hemiacetal carbon atom? A. B. C. D. enantiomers isomers epimers anomers 30 Question Cyclization of monosaccharides: A. B. C. D. is the reaction of hemiketal or hemiacetal formation. is irreversible. creates α and β epimers. only occurs in hexoses. 31 Haworth Perspective Formulas Haworth perspective formulas = more accurate representation of cyclic sugar structure than Fischer projections – six-membered ring is tilted to make its plane almost perpendicular to that of the paper – bonds closest to the reader are drawn thicker than those farther away 32 Converting D-Hexose Fischer Projections to Haworth Perspective Formulas step 1: draw the six-membered ring (five carbons, and one oxygen at the upper right) step 2: number the carbons in a clockwise direction beginning with the anomeric carbon step 3: place the hydroxyl groups – hydroxyl groups on the right in a Fischer projection are placed pointing down and those on the left are placed pointing up step 4: place the terminal —CH2OH group – projects upward for the D enantiomer, downward for the L enantiomer step 5: place the anomeric hydroxyl group – for a β structure, the hydroxyl group is placed on the same side of the ring as C-6 33 – for an α structure, it is placed on the opposite side 34 Question Using the Fischer projection of D-glucose, identify which statement is true of the Haworth perspective formula of α-D-glucopyranose. A. The terminal —CH2OH group projects downward. B. The anomeric hydroxyl is on the opposite side from C-6. C. The hydroxyl group on C-2 is placed pointing up. D. The cyclic structure is a five-membered ring. 35 Conformational Formulas of Pyranoses pyranose rings tend to assume either of two “chair” conformations – interconvertible without breaking covalent bonds – requires energy input 36 Organisms Contain a Variety of Hexose Derivatives 37 Aldonic and Uronic Acids aldonic acids = form following oxidation of the carbonyl carbon of aldoses uronic acids = form following oxidation at C-6 both form stable intramolecular esters called lactones 38 Phosphorylated Derivatives some sugar intermediates are phosphate esters – example: glucose 6-phosphate stable at neutral pH and bear a negative charge functions to trap sugar inside the cell because most cells do not have membrane transporters for phosphorylated sugars 39 Sugars That Are, or Can Form, Aldehydes Are Reducing Sugars reducing sugars = undergo a characteristic redox reaction where free aldehyde groups react with Cu2+ under alkaline condition – reduction of Cu2+ to Cu+ forms a brick-red precipitate ketoses that can tautomerize to form aldehydes are also reducing sugars 40 Question Which reaction is one that is NOT common of glucose? A. B. C. D. mutarotation oxidation to an aldonic acid oxidation to an uronic acid oxidation by reducing sugars 41 O-Glycosidic Bonds O-glycosidic bond = covalent linkage joining two monosaccharides – formed when a hydroxyl group of one sugar molecule reacts with the anomeric carbon of the other – readily hydrolyzed by acid 42 The Reducing End formation of a glycosidic bond renders a sugar nonreducing reducing end = the end of a disaccharide or polysaccharide chain with a free anomeric carbon Why do we use wavy lines? – Indicate that the structure may be α or β Free anomeric carbon 43 Question What term is given to carbohydrates linked by their anomeric carbons? A. B. C. D. nonreducing sugars glycosides anomers hemiketals 44 Naming Reducing Oligosaccharides step 1: with the nonreducing end on the left, give the configuration (α or β) at the anomeric carbon joining the first unit to the second step 2: name the nonreducing residue using “furano” or “pyrano” step 3: indicate in parentheses the two carbon atoms joined by the glycosidic bond, with an arrow connecting the two numbers step 4: name the second residue and repeat for additional residues 45 Naming Reducing Oligosaccharides step 1: with the nonreducing end on the left, give the configuration (α or β) at the anomeric carbon joining the first unit to the second step 2: name the nonreducing residue using “furano” or “pyrano” step 3: indicate in parentheses the two carbon atoms joined by the glycosidic bond, with an arrow connecting the two numbers step 4: name the second residue and repeat for additional residues 46 Question Following the convention for naming reducing oligosaccharides, what is the name of lactose, as shown. A. B. C. D. α-L-glucopyranose-(1→6)-α-L-galactopyranosyl β-D-glucopyranose-(1→4)-β-D-galactopyranosyl β-D-galactopyranosyl-(1→4)-β-D-glucopyranose α-L-galactopyranosyl-(1→6)-α-L-glucopyranose 47 Three Common Disaccharides lactose is a reducing disaccharide sucrose and trehalose are nonreducing sugars 48 Question Which of these sugars is nonreducing? A. B. C. D. trehalose glucose maltose lactose 49 Question Which item is NOT associated with disaccharides? A. B. C. D. O-glycosidic linkages formation of a ketal or acetal never having free anomeric carbons sucrose 50 What Makes Sugar Sweet? TAS1R2 and TAS1R3 encode sweet-taste receptors binding of a compatible molecule generates a “sweet” electrical signal in the brain – requires a steric match 51 Question Why does (R,S)-aspartame NOT stimulate the sweet receptor? A. Site AH+ cannot bind the partially negative oxygen of the carboxylic acid. B. Site B– cannot hydrogen bond with the amine nitrogen. C. Site X cannot accommodate the hydrophobic benzene ring of (R,S). D. None of the sites can interact with (R,S)-aspartame. 52