Question

One carbon groups from various sources can be used for the synthesis of new compounds. Outline the steps involved in the transfer of a one carbon group from glycine to homocysteine. How can the product of these reactions be used in further one carbon transfers?

Answer 1

The one-carbon (1C) groups play a key role in donating carbon atoms for synthesis of different types of compounds. The one-carbon groups are contributed to the one-carbon pool by amino acids. The different one-carbon groups are interconvertible.

Gycine undergoes oxidative deamination to form NH3, CO2 and the one carbon unit methylene THFA (Tetrahydrofolic acid). It involves amulti-enzyme complex consisting of glycine decarboxylase with pyridoxal phosphate, lipoamide containing amino methyl transferase, methylene THFA synthesizing enzyme and NAD+ dependent lipoamide dehydrogenase. The transfer of one carbon group from glycine to homocysteine involves the following steps:

(i) The amidino group of arginine is transferred to glycine to form guanidoacetic acid, catalyzed by amido transferase. Glycine + Arginine Guanidoacetic acid + Ornithine

(ii) Guanidoacetic acid is methylated by S-adenosyl methionine (SAM), which is formed from activation of methionine using enzymee methionine adenosyl transferase(MAT) to creatine. In this process methyl group is transferred which is catalyzed by enzyme methyl transferase with the formation of S-adenosyl homocysteine (SAH).

(iii) From the S-adenosyl homocysteine (SAH), the adenosyl group is removed to form homocysteine, catalyzed by adenosine homocysteinase enzyme.

The product ie Homocysteine formed by one carbon group transfer can be used further in one carbon transfers. Cysteine can be synthesized from homocysteine using one carbon transfer. The steps involved in it are as follows:

(i) Homocysteine formed condenses with serine to form cystathionine. This is catalyzed by pyridoxal phosphate dependent cystathionine-beta synthase. (Homocysteine + SerineCystathionine)

(ii) Cystathionine is hydrolyzed by cystathionase to form cysteine and homoserine.