Trends in Plant Science
ReviewThe glycine decarboxylase system: a fascinating complex
Section snippets
Structure of the glycine decarboxylase complex
The glycine decarboxylase multi-enzyme complex catalyses in a multi-step reaction the rapid ‘cracking’ of glycine molecules flooding out of the peroxisomes during the course of photorespiration (Fig. 1). As in its mammalian counterpart 13, the glycine decarboxylase multienzyme complex of green-leaf mitochondria consists of four different component enzymes, designated as the P-protein (a homodimer containing pyridoxal phosphate, 200 kDa), the H-protein (a monomeric lipoamide-containing protein,
Glycine decarboxylase-serine hydroxymethyl transferase coupled reactions
The glycine decarboxylase complex is connected to SHMT through a soluble pool of THF (Fig. 1). SHMT in pea leaf mitochondria is a 220 kDa homotetramer with a subunit molecular weight of 53 kDa (Refs 14,37). Each subunit contains a single pyridoxal-phosphate bound as a Schiff base to an ε-amino group of a lysine residue. If the C2-cycle is strictly cyclic, then all CH2–THF formed upon glycine oxidation must be used for serine synthesis and does not gain access to the general C1 pool. SHMT is
Metabolic control of glycine oxidation and substrate transport
Metabolic regulation of the glycine decarboxylase system might be affected by feedback inhibition by two of the final reaction products, serine and NADH (Refs 6,40). Serine inhibits in a competitive manner with glycine [Ki(serine)=4 mM; Km(glycine)=6 mM] the reaction catalysed by the P-protein 41. NADH competes with NAD+ for the active site on the L-protein [Km(NAD+)=75 μM; Ki(NADH)=15 μM] 14. This means that increasing the ratio of NADH to NAD+ in the matrix space should result in a
Glycine decarboxylase biogenesis
Mitochondrial SHMT (Ref. 37) and all four proteins of the GDC complex are encoded by nuclear genes 32, 33, 39, 45, 46, 47, 48, 49. The proteins of GDC are synthesized most abundantly in leaves, and at detectable but much lower levels in non-green tissues. The spectacular accumulation of glycine decarboxylase proteins in mitochondria from mesophyll cells of C3 plant leaves is probably attributable to a light-dependent transcriptional control of the genes encoding these proteins. Studies of
Biosynthesis of cofactors
Obviously, the formation of active GDC and SHMT is strictly dependent on an endogenous supply of various cofactors, including lipoic acid and THF. This raises the important problem of the origin of these cofactors 3. Plants and microorganisms, in contrast with animals (folate supply in these organisms is dependent on feeding), are able to synthesize THF from 6-hydroxymethyl-7,8-dihydropterin and p-amino benzoic acid (p-ABA) de novo. This pathway requires the sequential operation of five
Conclusions
Over the past several years, our understanding of the enzymes that impact upon glycine metabolism in plants has advanced dramatically. All the protein components (P-, H-, T- and L-proteins) of the glycine decarboxylase complex present in large amounts in the matrix space of green-leaf mitochondria have been purified and their genes cloned. This should help to further our understanding of the self recognition of these proteins in the association process. The structural and mechanistic heart of
Acknowledgements
Work on glycine decarboxylase is supported by the Centre National de la Recherche Scientifique (CNRS), the Commissariat à l'Energie Atomique (CEA) and the Ministère de la recherche. We would like to thank Claudine Cohen-Addad and David Macherel for their active collaboration during the course of this work.
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