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Practice%20Problems%20No_5 - of citrulline to ornithine in...

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of citrulline to ornithine in the cytosol. As such, ornithine and citrulline are transported into and out of, respectively, the mitochondrial matrix by their respective membrane transport systems. 2 . One amino group of urea originates from the ammonium ion, whereas the other one originates from aspartate’s alpha-amino group that can in turn originate from the alpha-amino groups of other standard amino acids via transamination reactions. 4. What are glucogenic amino acids, and ketogenic amino acids? Key: Glucogenic amino acids are those amino acids whose carbon skeletons can be degraded into metabolites (i.e. pyruvate, alpha-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate) that can be converted via gluconeogenesis to glucose. Ketogenic amino acids are those amino acids whose carbon skeletons can be degraded into metabolites (i.e. acetoacetate, acetyl-CoA) that can be converted to fatty acids. 5. How does the liver glucokinase play an important role in conferring the blood glucose “buffering” effect of liver? Key: The liver enzyme glucokinase catalyzes the phosphorylation of glucose to form glucose-6- phosphate (G6P). In contrast to its isozyme hexokinase, glucokinase has much weaker affinity for the substrate glucose, but its catalytic activity increases with [glucose] concentration within the normal physiological range. Furthermore, the activity of glucokinase is not inhibited by its reaction product glucose-6-phosphate (G6P) at physiological concentrations. Therefore, in response to an increased blood [glucose] and the subsequent glucose uptake by the liver cells, glucokinase activity rapidly increases and it converts glucose into G6P. (
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