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Old 07-22-2012
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Biochemistry Enzymatic changes in carb-rich diet

A 39-year-old man consumes a carbohydrate-rich diet. Which of the following best characterizes the enzyme that will be increased in this patient?

a) Catalyzes the conversion of glucose to glucose-6-phosphate in peripheral tissues
b) Catalyzes the rate-limiting step in glycolysis
c) Characterized by a high affinity (low Km) for glucose
d) Functions only when the glucose concentration in hepatocytes is relatively high
e) Inhibited by glucose-6-phosphate
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b. Insulin increases, increasing PFK activity.

edit:my bad...picked the wrong option. I meant B not A.

Last edited by 587964; 07-22-2012 at 10:22 AM.
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Old 07-22-2012
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Originally Posted by smanthrav View Post
A. Insulin increases, increasing PFK activity.
a) Catalyzes the conversion of glucose to glucose-6-phosphate in peripheral tissues - it's not PFK (phosphofruktokinase), it's hexokinase
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Default I think its D

d) Functions only when the glucose concentration in hepatocytes is relatively high = GLUCOKINASE
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hexokinase... Low Km (vs Glucokinase) so is C
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Quote:
Originally Posted by numbndumb View Post
d) Functions only when the glucose concentration in hepatocytes is relatively high = GLUCOKINASE
i would say the same.
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Old 07-28-2012
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What's the answer?
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Old 07-28-2012
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Correct Answer correct answer :)

The answer is d) Functions only when the glucose concentration in hepatocytes is relatively high

After a carbohydrate-rich diet is ingested, high levels of glucose are delivered to the liver via the portal vein. Levels of glucokinase are increased because of the high intracellular glucose concentration. Glucokinase has a low affinity (high Km) and high Vmax for glucose. It prevents hyperglycemia in the systemic circulation. The enzyme is located in the liver and pancreatic beta cells and is inducible by insulin.

Option A (Catalyzes the conversion of glucose to glucose-6-phosphate in peripheral tissues) is incorrect. The enzyme that catalyzes the conversion of glucose to glucose-6-phosphate in peripheral tissues is hexokinase. It is found in most tissues, but it does not respond to increased glucose levels. In contrast to glucokinase, which functions only when tissue concentrations of glucose are high, hexokinase functions when tissue concentrations of glucose are low (e.g., patient with hypoglycemia). It cannot phosphorylate large amounts of glucose. Characteristics include a low Michaelis constant (Km) (high affinity) and low maximum velocity (Vmax). It is not inducible by insulin.

Option B (Catalyzes the rate-limiting step in glycolysis) is incorrect. The rate-limiting step in glycolysis is catalyzed by phosphofructokinase 1, which converts fructose 6-phosphate to fructose 1,6-bisphosphate.

Option C (Characterized by a high affinity (low Km) for glucose) is incorrect. Glucokinase has a low affinity (high Km) for glucose.

Option E (Inhibited by glucose-6-phosphate) is incorrect. Glucokinase is not inhibited by glucose-6-phosphate, but it is inhibited by its isomer, fructose 6-phosphate. Hexokinase is inhibited by glucose-6-phosphate.
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