Structural biology of biotin-dependent carboxylases

生物素依赖性羧化酶使用共价连接的生物素辅因子作为羧基生物素来运输二氧化碳。四种人类生物素依赖性 羧化酶已被鉴定，并且在代谢途径如奇数链脂肪酸的氧化、支链氨基酸的催化、脂肪酸合成和脂肪异生中起中心作用。 细菌转羧酶长期以来一直作为人类生物不依赖性羧化酶的模型系统。该建议集中于转羧酶的结构-功能研究和两种人类酶，丙酰辅酶A羧化酶和甲基巴豆酰辅酶A 羧化酶广泛的目标是提供这些酶的结构信息，以便更好地了解它们作为多酶复合物的组装及其催化活性的机制。具体目标是：1.探讨转羧酶的化学性质和作用机制 活性位点与分离的12 S和5S亚基的结构研究。将通过X射线晶体学和拉曼晶体学研究与底物、产物或辅因子结合的分离亚基的晶体。定点诱变将用于探测结构研究所强调的残基的重要性。2.用X射线晶体学和电子显微镜研究完整的转羧酶多酶复合物的分子结构。3.研究人丙酰辅酶A羧化酶的晶体结构，并模拟代谢紊乱丙酸血症中缺陷突变的可能结构后果。4.研究晶体结构 的人甲基巴豆酰辅酶A羧化酶β亚基，并模拟可能的结构后果的缺乏患者的代谢紊乱3-甲基巴豆酰甘氨酸尿症。相关性：生物素依赖性酶在人体代谢中很重要。在患有代谢和发育障碍的患者中发现了改变其基因的突变。研究这些酶的结构将有助于了解这些蛋白质如何组装和发挥功能，并有助于解释突变如何导致疾病。

Biotin-dependent carboxylases use a covalently attached biotin cofactor to transport carbon dioxide as carboxybiotin. Four human biotin-dependent carboxylases have been identified, and play central roles in metabolic pathways such as oxidation of odd-chain fatty acids, catabolism of branched amino acids, fatty acid synthesis, and gluconeogenesis. The bacterial transcarboxylase enzyme has long served as a model system for the human biotindependent carboxylases. This proposal focuses on structure-function studies of transcarboxylase and on two human enzymes, propionyl coenzyme A carboxylase and methylcrotonoyl coenzyme A carboxylase. The broad objective is to provide structural information for these enzymes in order to better understand their assembly as multi-enzyme complexes and their mechanisms of catalytic activity. The specific aims are to: 1. To probe the chemistry and mechanism of the transcarboxylase active sites with structural studies of the isolated 12S and 5S subunits. Crystals of the isolated subunits bound to substrate, product, or cofactor will be studied by X-ray crystallography and Raman crystallography. Site- directed mutagenesis will be used to probe the importance of residues highlighted by structural studies. 2. To probe the molecular architecture of the intact transcarboxylase multienzyme complex by X-ray crystallography and electron microscopy. 3. To investigate the crystal structure of human propionyl coenzyme A carboxylase and to model the possible structural consequences of deficiency mutations in the metabolic disorder propionic acidemia. 4. To investigate the crystal structure of the human methylcrotonyl coenzyme A carboxylase beta subunit and to model the possible structural consequences of deficiency patients with the metabolic disorder 3-methylcrotonylglycinuria. Relevance: Biotin-dependent enzymes are important in human metabolism. Mutations which alter their genes are found in patients with metabolic and developmental disorders. Investigating the structures of these enzymes will aid understanding of how these proteins assemble and function, and help explain how mutations may cause disease.