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Chapter 23: Q15 P (page 1085)

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.

Short Answer

Expert verified

If the pyridine nitrogen is not protonated on to the positively charged protonated nitrogen of pyridine ring then it would be less attractive to electrons and thereby decreases enzymatic reactions.

Step by step solution

01

Reason behind less ability of PLP

Amino acid reacts with imine formed by PLP and the lysine side chain converts to another imine by transimination reaction. The next step after this reaction is breaking substituents from α- carbon of amino acid. The electrons left behind when bond breaks are delocalized on to the positively charged protonated nitrogen of pyridine ring. If the pyridine nitrogen is not protonated, then it would be less attractive to electrons. Hence, pyridine nitrogen less crowded with electrons.

02

Imine between PLP and amino acid

Here the structure represents an imine linkage in between PLP coenzyme and an amino acid containing hydrocarbon side chain.

Thus, the reason behind the reduction in PLP to catalyze an amino acid transformation is discussed above.

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Most popular questions from this chapter

Draw structures that show the similarity between the decarboxylation of the pyruvate–TPP intermediate and the decarboxylation of a \(\beta \)-keto acid.

Instead of adding to the 4a position and protonating N-5, the thiolate ion could have added to the 10a position and protonated N-1. (The numbering system is on page 1071.) Why is addition to the 4a position favored? (Hint:Which nitrogen is a stronger base?)

The enzyme that catalyzes the Cα-Cβbond 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.)

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?

Question:The glycine cleavage system is a group of four enzymes that together catalyze the following reaction:

glycine+THF→glycinecleavagesystemN5,N10-methylene-THF+CO2

Use the following information to determine the sequence of reactions carried out by the glycine cleavage system:

a. The first enzyme is a PLP-requiring decarboxylase.

b. The second enzyme is aminomethyltransferase. This enzyme has a lipoate coenzyme.

c. The third enzyme synthesizes N6,N10-methylene-THF and also forms NH4

d. The fourth enzyme is an FAD-requiring enzyme.

e.The cleavage system also requires NAD.
See all solutions

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