PXLP-GCAT dimer ligates CoASH to 2A-3OB to form Gly and Ac-CoA

The degradation of L-threonine to glycine in both prokaryotes and eukaryotes takes place through a two-step biochemical pathway. In the second step, mitochondrial 2-amino-3-ketobutyrate coenzyme A ligase (GCAT, aka KBL) catalyses the reaction between 2-amino-3-oxobutanoate (2A-3OBU) and coenzyme A (CoA-SH) to form glycine (Gly) and acetyl-CoA (Ac-CoA) (Edgar & Polak 2000). GCAT resides on the mitochondrial inner membrane and requires pyridoxal 5-phosphate (PXLP) as cofactor. It is strongly expressed in heart, brain, liver and pancreas. Dimeric GCAT:PXLP is thought to exist on the mitochondrial inner membrane in complex with tetrameric L-threonine 3-dehydrogenase (TDH), the first enzyme in this pathway (Tressel et al. 1986). With these two enzymes located together, it stops the rapid and spontaneous decarboxylation of 2A-3OBU to aminoacetone and carbon dioxide and instead, results in glycine formation.

在原核生物和真核生物中，L-苏氨酸的降解至甘氨酸的过程通过一个两步生化途径进行。在第二步中，线粒体中的2-氨基-3-酮丁酸辅酶A连接酶（GCAT，亦称KBL）催化2-氨基-3-氧丁酸（2A-3OBU）与辅酶A（CoA-SH）之间的反应，生成甘氨酸（Gly）和乙酰辅酶A（Ac-CoA）（Edgar & Polak 2000）。GCAT位于线粒体内膜上，并需要吡哆醛-5-磷酸（PXLP）作为辅因子。它在心脏、大脑、肝脏和胰腺中表达强烈。二聚体GCAT:PXLP被认为存在于线粒体内膜上，与四聚体L-苏氨酸3-脱氢酶（TDH）复合，该酶是该途径中的第一酶（Tressel et al. 1986）。这两酶的相邻定位，有效阻止了2A-3OBU快速自发脱羧生成氨基乙酮和二氧化碳，进而促成甘氨酸的生成。