Free FMGE Biochemistry questions

Explanation

Vitamin K is required for gamma-carboxylation of several coagulation factors. Deficiency consequently impairs normal coagulation.

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The clinical triad suggests Wernicke encephalopathy due to thiamine deficiency. Thiamine pyrophosphate is required for several oxidative decarboxylation reactions and transketolase.

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Ascorbate is needed for hydroxylation of proline and lysine during collagen maturation. Deficiency produces defective connective tissue and features of scurvy.

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Competitive inhibition can be overcome by increasing substrate concentration, so Vmax remains unchanged. More substrate is required to reach a given velocity, increasing apparent Km.

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Pure noncompetitive inhibition reduces the amount of functionally active enzyme and therefore lowers Vmax. Substrate-binding affinity is unchanged, leaving Km unchanged.

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PFK-1 converts fructose-6-phosphate to fructose-1,6-bisphosphate and is a major regulatory enzyme of glycolysis. Its activity reflects cellular energy status.

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Pyruvate is first converted to oxaloacetate and then to phosphoenolpyruvate by PEP carboxykinase. These reactions bypass the irreversible pyruvate kinase reaction.

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Von Gierke disease is glycogen storage disease type I caused by impaired glucose-6-phosphatase activity. The liver cannot efficiently release free glucose during fasting.

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The pentose phosphate pathway generates NADPH, which helps maintain reduced glutathione. This is particularly important for protecting erythrocytes from oxidative injury.

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HMG-CoA reductase catalyzes a major regulatory step in cholesterol synthesis. It is also the pharmacological target of statins.

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Long-chain fatty acids use the carnitine shuttle to cross the inner mitochondrial membrane. Defects in this process impair fatty-acid oxidation.

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The initial reactions of the urea cycle involving carbamoyl phosphate synthetase I and ornithine transcarbamylase occur in mitochondria. Later steps occur in the cytosol.

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Phenylalanine hydroxylase normally converts phenylalanine to tyrosine. Its deficiency causes accumulation of phenylalanine and its metabolites.

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Alkaptonuria results from deficiency of homogentisate oxidase in tyrosine degradation. Homogentisic acid accumulation causes dark urine and ochronotic pigmentation.

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DNA polymerases cannot initiate synthesis de novo. Primase generates an RNA primer providing the required free 3′ hydroxyl group.

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DNA polymerases add nucleotides to the 3′\-OH end of a growing strand. New DNA is therefore synthesized in the 5′ to 3′ direction.

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Xeroderma pigmentosum involves defective nucleotide-excision repair of UV-induced DNA damage. Patients consequently have marked photosensitivity and increased skin-cancer risk.

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Hemoglobin S contains valine instead of glutamate at position 6 of the beta-globin chain. This substitution promotes polymerization of deoxygenated HbS.

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Acute intermittent porphyria is caused by porphobilinogen deaminase deficiency. Neurologic and abdominal manifestations occur without the photosensitivity characteristic of several other porphyrias.

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Tay-Sachs disease results from hexosaminidase A deficiency with accumulation of GM2 ganglioside. A cherry-red spot with absence of hepatosplenomegaly helps distinguish it from Niemann-Pick disease.