Jackowski S, Rock C O
J Biol Chem. 1983 Dec 25;258(24):15186-91.
Acyl carrier protein (ACP) functions as a cofactor in fatty acid biosynthesis due to the covalent linkage of an acyl moiety to its 4'-phosphopantetheine prosthetic group. This prosthetic group undergoes turnover in vivo and since the apoprotein is functionally inactive, the interconversion between ACP and apo-ACP has been considered as a possible regulatory point in lipid biosynthesis. To investigate this possibility, the ratio of ACP to apo-ACP was measured in Escherichia coli. An apo-ACP standard was synthesized using [ACP] phosphodiesterase (EC 3.1.4.14) and could be clearly separated from ACP by conformationally sensitive gel electrophoresis, thus providing a reliable assay for the presence of these two species. Antibodies specific for ACP were purified from rabbit serum on an ACP-Sepharose column and subsequently used to synthesize an immunoaffinity column. Chromatography of leucine-labeled cell extracts on this support resulted in the specific binding of ACP, but apo-ACP was not detected in either logarithmically growing or stationary phase cells, although both ACP species bound to the purified anti-ACP IgG. Apo-ACP was not detected as an intermediate in ACP biosynthesis, suggesting that apo-ACP is rapidly converted to ACP following translation. CoA is the biosynthetic precursor to the ACP prosthetic group, but apo-ACP did not accumulate when the intracellular CoA concentration was severely depressed in strain SJ16 (panD), a beta-alanine auxotroph. Strain MP4 (acpS) is conditionally defective in [ACP]synthase (EC 2.7.8.7) and apo-ACP was the predominant form of ACP synthesized in this strain under nonpermissive conditions. Even under conditions that permitted growth, apo-ACP comprised 70% of the total ACP pool in strain MP4. Strain MP4 possessed a phospholipid to protein ratio within the normal range, suggesting that the ratio of ACP to apo-ACP can be significantly altered without affecting total lipid content. Thus, it appears that the prosthetic group turnover cycle maintains all of the ACP in an active form in vivo and a regulatory role for the ACP/apo-ACP ratio seems doubtful.
酰基载体蛋白（ACP）在脂肪酸生物合成中作为一种辅助因子发挥作用，这是由于其酰基部分与4'-磷酸泛酰巯基乙胺辅基发生共价连接。该辅基在体内会发生周转，且由于脱辅基蛋白功能失活，因此ACP与脱辅基ACP之间的相互转化被认为是脂质生物合成中一个可能的调控点。为了研究这种可能性，对大肠杆菌中ACP与脱辅基ACP的比例进行了测定。使用[ACP]磷酸二酯酶（EC 3.1.4.14）合成了脱辅基ACP标准品，并且通过构象敏感凝胶电泳可将其与ACP清晰分离，从而为这两种物质的存在提供了可靠的检测方法。从兔血清中在ACP-琼脂糖柱上纯化了对ACP具有特异性的抗体，随后用于合成免疫亲和柱。用亮氨酸标记的细胞提取物在该支持物上进行色谱分析，结果显示ACP特异性结合，但在对数生长期或稳定期细胞中均未检测到脱辅基ACP，尽管这两种ACP形式均与纯化的抗ACP IgG结合。未检测到脱辅基ACP作为ACP生物合成的中间体，这表明脱辅基ACP在翻译后会迅速转化为ACP。辅酶A是ACP辅基的生物合成前体，但在β-丙氨酸营养缺陷型菌株SJ16（panD）中，当细胞内辅酶A浓度严重降低时，脱辅基ACP并未积累。菌株MP4（acpS）在[ACP]合酶（EC 2.7.8.7）方面存在条件性缺陷，在非允许条件下，脱辅基ACP是该菌株中合成的ACP的主要形式。即使在允许生长的条件下，脱辅基ACP在菌株MP4的总ACP库中仍占70%。菌株MP4的磷脂与蛋白质比例在正常范围内，这表明ACP与脱辅基ACP的比例可以在不影响总脂质含量的情况下发生显著改变。因此，似乎辅基周转循环在体内使所有的ACP都保持在活性形式，而ACP/脱辅基ACP比例的调控作用似乎值得怀疑。
