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Acetolactatesynthasetransfers the acyl group of pyruvate to \(\alpha \)-ketobutyrate. This is the first step in the biosynthesis of the amino acid isoleucine. Propose a mechanism for this reaction.

a. What acyl group does pyruvate decarboxylase transfer to a proton?

b. What acyl group does the pyruvate dehydrogenase complex transfer to coenzyme A?

c. What acyl group does transketolase transfer to ribose-5-P?

Which compound is more easily decarboxylated?

\(\alpha \)-Keto acids other than \(\alpha \) -ketoglutarate can accept the amino group from pyridoxamine in enzyme catalyzed transamination reactions. What amino acids are formed when the following a-keto acids accept the amino group?

Explain why the ability of PLP to catalyze an amino acid transformation is greatly reduced if a PLP- requiring enzymatic reaction is carried out at a pH at which the pyridine nitrogen is not protonated.

Explain why the ability of PLP to catalyze an amino acid transformation is greatly reduced if the OH substituent of pyridoxal phosphate is replaced by OCH₃.

The enzyme that catalyzes the ${\mathit{C}}_{\mathbf{Î\pm }}\mathbf{-}{\mathit{C}}_{\mathbf{β}}$bond cleavage reaction that converts serine to glycine removes the substituent (R) bonded to the \(\alpha \)-carbon in the first step of the reaction. Starting with PLP bound to serine in an imine linkage, propose a mechanism for this reaction. (Hint:The first step involves removal of the proton from serine’s OH group.)

Propose a mechanism for the $\mathit{Î\pm }\mathbf{,}\mathit{β}$-elimination reaction shown on page 1082.

Question:A fatty acid (a long straight-chain carboxylic acid with an even number of carbons) is metabolized to acetyl- CoA, which can then enter the citric acid cycle to be further metabolized (Section 24.9). A fatty acid with an odd number of carbons is metabolized to acetyl-CoA and one equivalent of propionyl-CoA. Propionyl-CoA cannot enter the citric acid cycle. Two coenzyme-requiring enzymes are needed to convert it to succinyl-CoA, a compound that can enter the citric acid cycle. Write the two enzyme-catalyzed reactions and include the names

of the required coenzymes.

Question: How do the structures of tetrahydrofolate and aminopterin differ?