Heath R J, Rock C O
Department of Biochemistry, St Jude Children's Research Hospital, Memphis, Tennessee 38101, USA.
J Biol Chem. 1996 May 3;271(18):10996-1000. doi: 10.1074/jbc.271.18.10996.
beta-Ketoacyl-acyl carrier protein (ACP) synthase III (the fabH gene product) condenses acetyl-CoA with malonyl-ACP to initiate fatty acid biosynthesis in the dissociated, type II fatty acid synthase systems typified by Escherichia coli. The accumulation of malonyl-acyl carrier protein (ACP) following the inhibition of a reconstituted fatty acid synthase system by acyl-ACP implicated synthase III (FabH) as a target for acyl-ACP regulation (Heath, R. J., and Rock, C. O. (1996) J. Biol. Chem. 271, 1833-1836); therefore, the FabH protein was purified and its biochemical and regulatory properties examined. FabH exhibited a Km of 40 microM for acetyl-CoA and 5 microM for malonyl-ACP. FabH also accepted other acyl-CoAs as primers with the rank order of activity being acetyl-CoA approximately propionyl-CoA >> butyryl-CoA. FabH utilized neither hexanoyl-CoA nor octanoyl-CoA. Acyl-ACPs suppressed Fabh activity, and their potency increased with increasing acyl chain length between 12 and 20 carbon atoms. Nonesterified ACP was not an inhibitor. Acyl-ACP inhibition kinetics were mixed with respect to acetyl-CoA, but were competitive with malonyl-ACP, indicating that acyl-ACPs decrease FabH activity by binding to either the free enzyme or the acyl-enzyme intermediate. These data support the concept that the inhibition of chain initiation at the beta-ketoacyl-ACP synthase III step contributes to the attenuation of fatty acid biosynthesis by acyl-ACP.
β-酮脂酰-酰基载体蛋白（ACP）合酶III（fabH基因产物）将乙酰辅酶A与丙二酸单酰-ACP缩合，在以大肠杆菌为代表的解离型II型脂肪酸合酶系统中启动脂肪酸生物合成。在重组脂肪酸合酶系统被酰基-ACP抑制后，丙二酸单酰-酰基载体蛋白（ACP）的积累表明合酶III（FabH）是酰基-ACP调节的靶点（希思，R.J.，和罗克，C.O.（1996年）《生物化学杂志》271，1833 - 1836）；因此，纯化了FabH蛋白并检测了其生化和调节特性。FabH对乙酰辅酶A的Km为40微摩尔，对丙二酸单酰-ACP的Km为5微摩尔。FabH也接受其他酰基辅酶A作为引物，活性顺序为乙酰辅酶A≈丙酰辅酶A >> 丁酰辅酶A。FabH既不利用己酰辅酶A也不利用辛酰辅酶A。酰基-ACPs抑制FabH活性，其效力随着酰基链长度在12至20个碳原子之间增加而增强。未酯化的ACP不是抑制剂。酰基-ACP抑制动力学对于乙酰辅酶A是混合型的，但对于丙二酸单酰-ACP是竞争性的，表明酰基-ACPs通过与游离酶或酰基-酶中间体结合来降低FabH活性。这些数据支持这样的概念，即在β-酮脂酰-ACP合酶III步骤对链起始的抑制有助于酰基-ACP对脂肪酸生物合成的衰减作用。
