Carbohydrate Structure and Metabolism (1D) - Bio & Biochem

1. What is an anomer in carbohydrate chemistry?

   Stereoisomers differing at the anomeric carbon after cyclization. Results from the creation of a new chiral center upon ring closure, yielding α and β forms.

2. What is mutarotation in aqueous solution?

   Interconversion of $b$ and $b$ anomers via the open-chain form. Enables equilibrium between cyclic forms in solution, resulting in a change in optical rotation.

3. What enzyme converts pyruvate to acetyl-CoA, and what cofactors are required?

   Pyruvate dehydrogenase; TPP, lipoate, FAD, NAD$^+$, CoA. Facilitates entry into the TCA cycle via a multienzyme complex using these cofactors for decarboxylation and CoA attachment.

4. What enzyme bypasses pyruvate kinase in gluconeogenesis (final step to PEP)?

   PEP carboxykinase (with pyruvate carboxylase upstream). Bypasses the irreversible pyruvate kinase step by forming PEP from oxaloacetate in two enzyme reactions.

5. What is the committed step of glycolysis and its enzyme?

   Fructose-$6$-phosphate $d$ fructose-$1,6$-bisphosphate; PFK-$1$. Marks the regulatory committed step in glycolysis, catalyzed by phosphofructokinase-1 under allosteric control.

6. What structural feature makes a sugar a reducing sugar?

   A free anomeric carbon that can form an open-chain aldehyde/ketone. Allows the sugar to tautomerize and expose a carbonyl for oxidation, enabling reduction of reagents like Cu$^{2+}$.

7. What does the D/L designation of a monosaccharide refer to in a Fischer projection?

   Configuration at the highest-numbered chiral carbon vs D/L glyceraldehyde. Assigns the sugar to the D or L series based on similarity to glyceraldehyde enantiomers at the penultimate chiral carbon.

8. Which disaccharide is nonreducing because both anomeric carbons are in the bond?

   Sucrose. Features a glycosidic bond tying both anomeric carbons, preventing opening to a reducing form.

9. What functional group defines an aldose monosaccharide (rather than a ketose)?

   Aldehyde group at the terminal carbon (usually $C_1$). Determines the classification as an aldose, contrasting with ketoses that have an internal ketone group.

10. What glycolysis step produces NADH, and which enzyme catalyzes it?

    Glyceraldehyde-$3$-phosphate $d$ 1,3-BPG; GAPDH. Oxidizes the aldehyde to carboxylic acid, generating NADH in the payoff phase of glycolysis.

11. What is the net energy cost of gluconeogenesis to make $1$ glucose from $2$ pyruvate?

    Consumes $4$ ATP, $2$ GTP, and $2$ NADH. Offsets the ATP yield of glycolysis and provides reducing power for biosynthetic reactions.

12. In a Fischer projection, what determines whether a sugar is D (not L)?

    The OH on the highest-numbered chiral carbon is on the right. Mirrors the configuration of D-glyceraldehyde in Fischer projections for assignment to the D series.

13. What is the net yield of glycolysis per glucose under aerobic conditions?

    Net $2$ ATP, $2$ NADH, and $2$ pyruvate. Represents the energy and reducing power gained from glucose breakdown before pyruvate oxidation.

14. What enzyme converts pyruvate to lactate during anaerobic glycolysis?

    Lactate dehydrogenase. Reduces pyruvate using NADH, regenerating NAD$^+$ to continue anaerobic ATP production.

15. In a Haworth projection of a D-sugar, what defines the $b$ anomer?

    Anomeric OH is on the same side as the $CH_2OH$ group (cis). Identifies the β configuration in D-sugars where anomeric OH and CH$_2$OH are cis in the ring plane.

16. Which glycolysis steps are irreversible and what enzymes catalyze them?

    Hexokinase, PFK-$1$, and pyruvate kinase steps are irreversible. Require bypassing in gluconeogenesis due to unfavorable thermodynamics in the reverse direction.

17. What is the rate-limiting enzyme of the pentose phosphate pathway oxidative phase?

    Glucose-$6$-phosphate dehydrogenase (G6PD). Controls NADPH production for biosynthesis and antioxidant defense in the irreversible oxidative branch.

18. What is the rate-limiting enzyme of gluconeogenesis?

    Fructose-$1,6$-bisphosphatase. Regulates gluconeogenesis by reversing the PFK-1 step, controlled by energy status and hormones.

19. What is the primary purpose of fermentation in human cells?

    Regenerate NAD$^+$ from NADH to sustain glycolysis. Maintains glycolytic flux under anaerobic conditions by recycling the electron acceptor for GAPDH.

20. What functional group defines a ketose monosaccharide (rather than an aldose)?

    Ketone group on an internal carbon (usually $C_2$). Determines the classification as a ketose, contrasting with aldoses that have a terminal aldehyde group.

21. What is the anomeric carbon in a cyclic monosaccharide?

    The carbonyl carbon in the open-chain form (hemiacetal/hemiketal carbon). Serves as the site of hemiacetal or hemiketal formation during cyclization of the sugar.

Study these flashcards