CHEM 153C Midterm: CMP Pathway study questions

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Central Metabolic Pathways
Glycolysis
1. The central metabolic pathways are said to be amphibolic. What is meant by this statement?
They are involved in both biosynthetic and catabolic metabolism.
2. The tricarboxylic acid cycle is amphibolic. What is meant by this term?
Amphibolic pathways are involved in both anabolism (biosynthesis) and catabolism (degradation). The TCA cycle is
involved in catabolism to generate energy in the form of ATP and intermediates needed for biosynthesis (e.g. alpha-
ketoglutarate and oxaloacetate).
3. Uptake of glucose from the blood must be appropriate to the needs of various tissue cells. What limits glucose uptake from
the blood? How do cells control the amount of glucose they take up?
Uptake of glucose depends on the difference in glucose concentration between the blood and the cytosol. The cytosolic
[glucose] is reduced by phosphorylating glucose to glucose-6-P. If glucose-6-P is not metabolized it inhibits Hexokinase
resulting in elevation of cytosolic [glucose] and reduction in further glucose uptake.
4. Phosphoglucose Isomerase catalyzes the following reaction in glycolysis:
How is this reaction important for the metabolism of glucose?
The isomerization of glucose-6-P to fructose-6-P moves the carbonyl (C=O) from C1 to C2 facilitating the symmetrical
aldol cleavage to two 3-carbon sugars.
Fructose-6-P provides an alternative substrate for production of erythrose-4-P and ribose-5-P via the reversible reactions
of the pentose phosphate pathway.
5. (a) Name the enzyme catalyzing the rate-limiting step in glycolysis.
Phosphofructokinase
(b) Briefly indicate what is meant by a rate-limiting step.
The step or reaction whose rate determines the rate of an entire pathway i.e. the slowest step.
(c) Predict the effect (positive or negative) of ADP on the catalytic properties of this enzyme.
Positive effector.
6. Triose Phosphate Isomerase catalyzes the following reaction in glycolysis:
Why is this reaction important for glycolysis?
Provides single 3-carbon sugar for further metabolism by glycolysis can deal with both fragments of cleavage using a
single set of enzymes.
7. The following reactions are important steps in glycolysis:
(a) Identify the enzyme(s) catalyzing these reactions.
Reaction 1: Glyceraldehyde-3-P Dehydrogenase; Reaction 2: Phosphoglycerate Kinase.
(b) Which reaction (1 or 2) illustrates the concept of substrate level phosphorylation?
Reaction 2.
(c) Justify your answer to (b) above.
Substrate level phosphorylation involves transfer of phosphate form phosphorylated intermediate to ADP to form ATP.
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8. Glyceraldehyde-3-P Dehydrogenase catalyzes the following reaction:
Glyceraldehyde-3-P + NAD+ + Pi 1,3-Bisphosphoglycerate + NADH + H+
Describe how this reaction illustrates the following general biochemical principles or actions.
(a) Oxidation of an organic substrate
Oxidation is the loss of electrons. In this case, glyceraldehyde-3-P aldehyde is oxidized to a carbonyl forming acyl-
phosphate, 1,3-bisphosphoglycerate, with the transfer of electrons to electron acceptor NAD+.
(b) Coupled reaction
The oxidation reaction described above is accompanied (coupled) to a phosphorylation reaction forming a high energy
acyl phosphate, 1,3-bisphosphoglycerate i.e. the energy released by oxidation is used to drive the formation of the
high energy acyl phosphate.
How does this reaction contribute to ATP production in glycolysis?
The high energy acyl phosphate, 1,3-bisphosphoglycerate, has sufficient free energy to transfer the phosphate to ADP to
form ATP in a subsequent substrate-level phosphorylation reaction.
9. Glyceraldehyde-3-P Dehydrogenase catalyzes the following reaction in glycolysis:
Briefly describe the significance of this reaction in energy metabolism.
Captures energy of oxidation for ATP production by coupling oxidation to formation of acyl phosphate which can
transfer a phosphate to ADP to form ATP by substrate level phosphorylation.
Reduces NAD+ to NADH whose oxidation by electron transport and oxidative phosphorylation results in ATP production.
10. Enolase catalyzes the following reaction in glycolysis:
Why is this reaction important for glycolysis?
Creates phosphorylated metabolite whose hydrolysis is sufficiently favorable to drive the formation of a
phosphoanhydride bond in ATP in the subsequent reaction.
11. Consider the following reactions of glycolysis:
3-Phosphoglycerate is the source of the amino acids serine, cysteine, and glycine and phosphoenolpyruvate is the source of
aromatic amino acids. Although Phosphoglycerate Mutase is a metabolic branch point, it is not subject to metabolic
regulation. Briefly discuss why this enzyme is unlikely to be regulated.
The reaction is near equilibrium and is therefore an inappropriate site for regulation; in additions, PEP is the precursor of
essential metabolites and needs to be produced under all conditions.
12. Pyruvate Kinase is an important enzyme of glycolysis. Phosphoenolpyruvate + ADP Pyruvate + ATP
Its activity is enhanced (increased) by fructose-1,6-bisP (FBP), an earlier intermediate of the glycolytic pathway. Provide a
logical explanation for
(a) why this enzyme should be regulated
It commits PEP, a compound with multiple fates, to a specific fate. [PEP is also used in the synthesis of aromatic amino
acids]. Enzyme is exergonic.
(b) why FBP is an appropriate positive regulator for the enzyme.
FBP is produced by phosphofructokinase, the key enzyme controlling the rate of glycolysis. Thus, the level of FBP is
directl related to a cell’s eeds for glycolysis. By activating pyruvate kinase, FBP insures that PEP is used primarily by
glycolysis.
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