D'Agnolo G, Rosenfeld I S, Vagelos P R
J Biol Chem. 1975 Jul 25;250(14):5289-94.
Two forms of beta-ketoacyl-acyl carrier protein (ACP) synthetase (designated I and II) have been identified in extracts of Escherichia coli. Synthetase I corresponds to the condensing enzyme that was studied earlier (GREENSPAN, M.D., ALBERTS, A.W., and VAGELOS, P.R. (1969) J. Biol. Chem. 244, 6477-6485); synthetase II represents a new form of the enzyme. Synthetase II was isolated as a homogeneous protein. It differs from synthetase I in having a higher molecular weight (76,999 versus 66,000), a lower pH optimum (5.5 to 6.1 versus 7.2), and a greater resistance to denaturation by heat. Synthetase II is similar to synthetase I in that both are inactivated by iodoacetamide, and prior incubation of the enzymes with fatty acyl thioesters prevents the inhibitory effect of iodoacetamide. Both also react with a fatty acyl thioester to form an acyl-enzyme intermediate, and the latter reacts with malonyl-ACP to form a beta-ketoacyl thioester. Specificity studies indicated that synthetase II, like synthetase I, has similar affinities with saturated and cis unsaturated fatty acyl thioesters of ACP that are intermediates in the synthesis of saturated and unsaturated fatty acids, respectively. The two synthetases differ only with respect to reactivity with palmitoleyl thioesters: synthetase II has a lower Km and higher Vmax than synthetase I with palmitoleyl-ACP. This finding suggests that synthetase II functions specifically in the elongation of palmitoleyl-ACP to form cis-vaccenyl-ACP. An investigation of synthetases I and II in two classes of unsaturated fatty acid auxotrophs revealed that synthetase I is absent in one class, fabB. Addition of wild type synthetase I to fabB fatty acid synthetase, which synthesizes only saturated fatty acids, permitted this fatty acid synthetase to synthesize unsaturated fatty acids. These experiments indicate that synthetase I plays a critical role in the synthesis of unsaturated fatty acids.
在大肠杆菌提取物中已鉴定出两种形式的β-酮脂酰-酰基载体蛋白（ACP）合成酶（分别命名为I和II）。合成酶I相当于早期研究的缩合酶（格林斯潘，医学博士，阿尔伯茨，A.W.，和瓦格洛斯，P.R.（1969年）《生物化学杂志》244，6477 - 6485）；合成酶II代表该酶的一种新形式。合成酶II被分离为一种均一的蛋白质。它与合成酶I的不同之处在于分子量更高（76,999对66,000）、最适pH更低（5.5至6.1对7.2）以及对热变性的抵抗力更强。合成酶II与合成酶I相似，二者都被碘乙酰胺灭活，并且在酶与脂肪酰硫酯预先温育可防止碘乙酰胺的抑制作用。二者还都与脂肪酰硫酯反应形成酰基 - 酶中间体，后者再与丙二酸单酰 - ACP反应形成β - 酮脂酰硫酯。特异性研究表明，合成酶II与合成酶I一样，对分别作为饱和脂肪酸和不饱和脂肪酸合成中间体的饱和及顺式不饱和脂肪酰硫酯具有相似的亲和力。这两种合成酶仅在与棕榈油酰硫酯的反应性方面存在差异：与棕榈油酰 - ACP反应时，合成酶II的Km值更低，Vmax值更高。这一发现表明合成酶II在将棕榈油酰 - ACP延长形成顺式 - vaccenyl - ACP的过程中具有特定功能。对两类不饱和脂肪酸营养缺陷型菌株中的合成酶I和II进行的研究表明，在一类fabB菌株中不存在合成酶I。向仅合成饱和脂肪酸的fabB脂肪酸合成酶中添加野生型合成酶I，可使该脂肪酸合成酶合成不饱和脂肪酸。这些实验表明合成酶I在不饱和脂肪酸的合成中起关键作用。
