Q36P Question:聽The glycine cleavage ... [FREE SOLUTION]

Chapter 23: Q36P (page 1098)

Question:The glycine cleavage system is a group of four enzymes that together catalyze the following reaction:
$glycine + THF \overset{glycine cleavage system}{鈫�} N^{5}, N^{10} - methylene - THF + {CO}_{2}$

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.

Short Answer

Expert verified

Glycine cleavage system helps to catalyze glycine to carbon dioxide, ammonia and N⁵ ,N1⁰
- methylene-tetrahydrofolate.

Step by step solution

Transamination reaction

Glycine cleavage consists of four enzymes which are glycine decarboxylase, aminomethyl transferase, dihydrolipoyl dehydrogenase and a carrier protein. In the first reaction, glycine undergoes transamination reaction using Pyridoxal phosphate. The enzyme used here is glycine decarboxylase. Carbon dioxide gets eliminated.

Step 2: Attack of 3-methyl-1,2-dithiolane

3-methyl-1,2-dithiolane attacks on the double bond between carbon and nitrogen. There occurs the transamination reaction using second enzyme, i.e. aminomethyl transferase. Thus it forms 4-aminomethylsulfanyl butane-2-thiol. This is shown below:

Usage of third enzyme, dihydrolipoyl dehydrogenase

The obtained product is treated with tetrahydrofuran using dihydrolipoyl dehydrogenase forming butane-1,3-dithiol, ammonium ion and methylene-tetrahydrofolate. It is shown below:

Usage of fourth enzyme

Butane-1,3-dithiol is treated with FAD,flavin adenine dinucleotide,using the fourth enzyme called carrier protein. It produces 3-methyl-1,2-dithiolane and FADH₂ which is then treated with NAD producing FAD, NADH and a proton.

So the complete mechanism is shown below:

So the sequence of reactions carried out by the glycine cleavage system is shown.