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Biochemistry - 3 Questions

1
Biochemistry

Which of the following enzymes is active in adipocytes following a heavy meal?

1) Glycogen phosphorylase
2) Glycerol kinase
3) Hormone-sensitive triacylglyceride lipase
4) Phosphatidate phosphatase

📝 Explanation:

The enzyme phosphatidate phosphatase converts phosphatidic acid to diacylglycerol during synthesis of triacylglycerides.

The function of adipose tissue is the storage of fatty acids as triacylglycerols in times of plenty and the release of fatty acids during times of fasting or starvation.

Fatty acids taken in by adipocytes are stored by esterification to glycerol-3-phosphate. Glycerol-3-phosphate is derived almost entirely from the glycolytic intermediate dihydroxyacetone phosphate through the action of glycerol-3-phosphate dehydrogenase. Glycolytic enzymes are active in adipocytes during triglyceride synthesis, but those of glycogen degradation (low levels in adipocytes) and gluconeogenesis (ie, glucose-6-phosphatase) are not.

Glycerol kinase is not present to any great extent in adipocytes, so that glycerol freed during lipolysis is not used to reesterify the fatty acids being released.

The enzyme triacylglyceride lipase is turned on by phosphorylation by a cyclic AMP-dependent protein kinase following epinephrine stimulation.

2
Biochemistry
Epinephrine causes increased blood glucose level due to:
1. Increased glycogenolysis in liver and muscle
2. Activation of phosphorylase
3. Inhibition of glycogen synthesis in liver
4. All of the above

📝 Explanation:

Epinephrine causes increased blood glucose level due to: 1. Increased glycogenolysis in liver and muscle 2. Activation of phosphorylase 3. Inhibition of glycogen synthesis in liver

3
Biochemistry

A vitamin B12 deficiency may affect heme synthesis by reducing the concentration of which of the following? Choose the one best answer.

1) Acetyl-CoA
2) Succinyl-CoA
3) Glycine
4) Alanine

📝 Explanation:

Vitamin B12 participates in two reactions in the body-conversion of homocysteine to methionine and conversion ofmethylmalonyl-CoA to succinyl-CoA.

Methylmalonyl-CoA is produced via various amino acid degradation pathways, and from oddcarbon chain fatty acid oxidation. In the absence of B12, succinyl-CoA would only be produced as an intermediate of the TCA cycle, and if it were removed from the cycle for heme synthesis, energy production may suffer.

Glycine is usually obtained from the diet, although in a B12 deficiency a functional folate deficiency may also develop, leading to an inhibition of serine hydroxymethyltransferase, the enzyme that converts serine to glycine, and requires free tetrahydrofolate.

Succinyl-CoA and glycine are the precursors for heme synthesis. A B12 deficiency would not inhibit the production of acetyl-CoA, succinate, or alanine. 

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