The mitochondrial glycine cleavage system. Functional association of glycine decarboxylase and aminomethyl carrier protein

Hiraga, K.; Kikuchi, G.

Journal of Biological Chemistry 255(24): 11671-11676


ISSN/ISBN: 0021-9258
PMID: 7440563
Accession: 006719094

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The activity of the purified glycine decarboxylase (tentatively called P-protein) which catalyzed the exchange of the glycine carboxyl carbon with CO2 was increased > 100,000 times by the addition of the aminomethyl carrier protein (tentatively called H-protein) and the Km for glycine was reduced to about one-fourth of that in the absence of H-protein. Decarboxylation of glycine was also greatly stimulated by the addition of H-protein and the decarboxylation yielded the H-protein-bound aminomethyl moiety and CO2 at a stoichiometric ratio of unity. P-protein and H-protein formed a fairly stable complex which could be demonstrated by gel filtration and by sucrose density gradient centrifugation. H-protein caused a significant change in the absorption spectrum of P-protein and a titration experiment indicated that 2 molecules of H-protein bind to 1 molecule of P-protein or 1 molecule of H-protein to each subunit of P-protein. H-protein also acted to reduce the Kd for methylamine, as estimated by the degree of spectral change of P-protein caused by the addition of methylamine, from 63-27 mM and the Kd value of 27 mM was practically equal to the Ki value (25 mM) for methylamine obtained in the glycine-CO2 exchange reaction in the presence of H-protein. H-protein seems to bring about a conformational change of P-protein which may be relevant to the expression of the decarboxylase activity of P-protein. Although P-protein is, by nature, glycine decarboxylase, the functional glycine decarboxylase may be an enzyme complex composed of P-protein and H-protein. H-protein seems to play a dual role in the glycine decarboxylation; the one as a regulatory protein of P-protein and the other as an electron-pulling agent and concomitantly as a carrier of the aminomethyl moiety derived from glycine by decarboxylation.