The mitochondrial glycine cleavage system. Purification and properties of glycine decarboxylase from chicken liver mitochondria

Hiraga, K.; Kikuchi, G.

Journal of Biological Chemistry 255(24): 11664-11670

1980


ISSN/ISBN: 0021-9258
PMID: 7440562
Accession: 006719096

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Abstract
Glycine decarboxylase, tentatively called P-protein and considered a constituent of the glycine cleavage system, was purified to apparent homogeneity from chicken liver mitochondria. P-protein is a homodimer having a MW = .apprx. 200,000 and consisting of identical subunits with MW = .apprx. 100,000. Each subunit apparently contains an equimolar pyridoxal 5'-phosphate which is bound to the protein, possibly through a protonated aldimine linkage. The isoelectric point of P-protein was 7.2. P-protein could bind glycine, showing a Kd of 33 mM for it, and could catalyze glycine decarboxylation even though the rate of decarboxylation catalyzed by P-protein alone was extremely low. The product of glycine decarboxylation was methylamine and the Km for glycine was .apprx. 40 mM, which is close to the Kd for glycine. Methylamine could bind to P-protein, giving a Kd value of 63 mM, and it inhibited the glycine decarboxylation. P-protein alone could slightly catalyze the exchange of carboxyl carbon of glycine with CO2; the exchange appeared to obey a ping-pong mechanism. Both glycine decarboxylation and the glycine-CO2 exchange catalyzed by P-protein were stimulated 100-fold or more by the addition of lipoic acid, which is a functional group of H-protein. P-protein may be defined as glycine decarboxylase, although P-protein alone exhibits only very low catalytic activities.