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Biochemical screening is performed in a mental institution for possible cases of retardation related to single enzyme defects. A mildly retarded individual is identified who has abnormally high urine levels of glutathione. Further examination demonstrates undetectable gamma-glutamyltranspeptidase activity in cultured skin fibroblasts. This enzyme is important in which of the following cellular functions?
A. Amino acid transport.
B. Cholesterol transport.
C. Fatty acid transport.
D. Nucleotide transport.
E. Sugar transport.
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i dont know if this is relevant for step 1... !!!
The correct answer is A. In addition to being important as a natural antioxidant that restores cysteinyl sulfhydryl groups on enzymes, glutathione also plays an important role in the gamma-glutamyl cycle that provides one mechanism for transporting amino acids into cells. Glutathione is actually a tripeptide, gamma-glutamylcysteinylglycine.
This peptide is unusual because one of the linkages is not the usual peptide linkage between a carboxyl group on one amino acid and an amino group on a second amino acid. Instead, in glutathione, there is a bond from the second, end (gamma), carboxyl group of the glutamate to the adjacent alpha amino group on cysteine. This leaves the glutamyl end of glutathione looking like an unattached amino acid, with both the ammonia and carboxyl groups free. This free end is used by the cell for "catching" amino acids outside the cell by binding them to the glutathione.
The enzyme that does this is gamma-glutamyltranspeptidase, illustrated in the question stem. The captured amino acids are then passed across the cell membrane to be released intracellularly by a second enzyme, gamma-glutamylcyclotransferase. A number of additional steps recreate the glutathione.
Deficiency of gamma-glutamyltranspeptidase produces mental retardation with increased plasma and urine levels of glutathione. Since alternative mechanisms of transporting amino acids are intact, the serum and urinary concentrations of individual amino acids remain normal in these patients, although the glutathione can be picked up as an "amino acid" on non-specific screens for amino acids.
Cholesterol (choice B) is transported in lipoproteins in the serum and as part of the membranes of vesicles or in lipid droplets intracellularly.
Fatty acids (choice C) are transported as triglycerides in lipoproteins and chylomicrons. They are released as free fatty acids that are picked up directly by a variety of peripheral tissues.
Nucleotides (choice D) are synthesized within cells rather than transported across the plasma membrane.
Sugar transport (choice E) involves a variety of enzymes on the intestinal brush border that specialize in breaking down disaccharides such as lactose and sucrose, and transporting glucose, galactose, and fructose.
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hi raji321
really a good question.
what is source of your questions?
thanks
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A.
thanks for the question....
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hi hamraj.. u r w..
these are from kaplan qbank - analysis.. i think 2004, not sure.. these r in my kaplan cd's
i have already posted them( 40 blocks) in the forum, if u want u can download them...
GL
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thanks raji321
wish you good luck
