Biochemistry 600 Quiz 11

In the formation of urea from ammonia, all of the following are correct EXCEPT; a. Aspartate supplies one of the nitrogens found in urea b. This is an energy-expensive process, utilizing several ATPs c. The rate of the cycle fluctuates with the diet d. Fumarate is produced e. The formation of citrulline is the rate-limiting step

A-D are correct. The rate limiting step in the urea cycle is the CPS1-catalyzed reaction. Correct response is E.

The major amino acid that is released from muscle and converted to glucose in the liver is a. Alanine b. Glutamate c. Aspartate d. Valine

Alanine is the carrier of ammonia produced in muscle (glucose-alanine cycle). Correct response is A.

Aminotransferases a. Usually require α-ketoglutarate or glutamine as one of the reacting pairs b. Catalyze irreversible reactions c. Require pyridoxal phosphate (PLP) as an essential cofactor for the reaction d. Are not able to catalyze transamination reactions with essential amino acids

Aminotransferases usually require α-ketoglutarate and glutamate as a reacting pair (please note: glutamate and not glutamine). Transamination reactions are reversible reactions. Aminotransferases can catalyze reactions involving essential amino acids as well. The reverse reaction is limited by insignificant levels of α-keto acids for essential amino acids. Correct response is C.

Carbomoyl phosphate synthetase 1 a. Is controlled primarily by feedback inhibition b. Is unresponsive to changes in arginine c. Requires N-acetyl glutamate as an allosteric regulator d. Requires ATP as an allosteric regulator

CPS1 is not controlled by feedback inhibition. It is activated by N-Acetyl Glutamate. The synthesis of N-acetyl Glutamate is activated by arginine. ATP is utilized during the urea cycle but does not regulate CPS1. Correct response is C.

True or false, Dietary amino acids absorbed from the intestinal brush border are transported as chylomicrons in blood for further processing in the liver.

Chylomicrons are used in the transportation of fatty acids and not amino acids. Statement is wrong.

The production of ammonia in the reaction catalyzed by glutamate dehydrogenase a. Requires the participation of NADH or NADPH b. May be reversed to consume ammonia if it is present in excess c. Is stimulated by high levels of ATP or GTP d. Would be inhibited when gluconeogenesis is active

Glutamate dehydrogenase is the only enzyme that can use both NAD+ and NADP+ (NOT NADH or NADPH). Glutamate dehydrogenase is inhibited by ATP or GTP. One of the major reason for amino acid catabolism is to provide metabolites for gluconeogenesis, as a result when glutamate dehydrogenase is active, gluconeogenesis is also likely to be active. The correct response is B.

True or false, Glutamate dehydrogenase can utilize both NAD+ and NADP+ as it catalyzes the formation of ammonia from glutamate.

Statement is True

True or False transamination reactions result in net use or production of amino acids.

Transamination results in formation of another amino acids using the α-keto acid that receives the amino group. The first amino acid that loses the amino group leaves its carbon skeleton. As a result, there is no net production or loss of amino acids (since one is lost but another one is formed). Statement is false.

True or false, Urea cycle is an entirely cytosolic process

Urea cycle occurs in two locations, mitochondria and cytosol. Statement is false.

True or false, Both proteosomal and lysosomal degradation of proteins require ubiquitination of the protein.

While proteosomal degradation of proteins require ubiquitination, lysosomal degradation is non selective under normal circumstances and does not require ubiquitination. Statement is false.