ADP-forming acetyl-CoA synthetases, functional and structural characterization of novel protein families in archaea and bacteria

最近，我们在古细菌中发现了一种新的酶，一种形成ADP的乙酰辅酶a合成酶（ACD），它可以催化乙酰辅酶a转化为乙酸，并将该反应与ATP的合成偶联（乙酰辅酶a + ADP + PI±+乙酸+ ATP + HS-CoA）。这种单酶形成醋酸的机制在原核生物中是不寻常的，似乎是古细菌域特有的，因为迄今为止分析的所有醋酸形成细菌都利用经典的通过磷酸转乙酰酶（PTA）和醋酸激酶（AK）形成醋酸的双酶机制。然而，许多利用AK/PTA的细菌含有功能未知的ACD同源物。ACD酶属于新近发现的形成ndp的酰基辅酶a合成酶的蛋白质超家族，其中还包括琥珀酰辅酶a合成酶（SCSs）。在这个超家族中，古细菌ACD和细菌ACD同源物构成了不同的亚家族，这表明细菌ACD同源物的结构域特异性作用不同于醋酸盐的形成。与已被很好地表征的SCSs相比，迄今为止还没有关于古菌和细菌中acd样酶的结构和机制的信息。尽管ACDs和scs之间存在相似之处，但在氨基酸序列和结构域结构上存在一些差异。此外，大多数细菌ACD同源物含有未知功能的额外乙酰转移酶结构域，本项目将对未知细菌ACD同源物进行表征，以确定其推测的生理功能。进一步分析了ACDs的反应机理和结构，并与SCS进行了比较。这些研究将扩展我们对中央古细菌和细菌代谢中一个不寻常的蛋白质家族的认识。

Recently, we identified a novel enzyme in archaea, an ADP-forming acetyl-CoA synthetase (ACD) that catalyses the conversion of acetyl-CoA to acetate and couples this reaction with the synthesis of ATP (acetyl-CoA + ADP + PI ±+ acetate + ATP + HS-CoA). This one-enzyme mechanism of acetate formation is unusual in prokaryotes and appears to be specific to the archaeal domain since all acetate forming bacteria analysed so far utilize the classical two-enzyme mechanism via phosphotransacetylase (PTA) and acetate kinase (AK). However, many of these AK/PTA utilizing bacteria contain ACD homologs of unknown function. ACD enzymes belong to the recently recognized protein superfamily of NDP-forming acyl-CoA synthetases, which also include succinyl-CoA synthetases (SCSs). Within this superfamily, archaeal ACDs and the bacterial ACD homologs constitute distinct subfamilies indicating a domain specific role of bacterial ACD homologs different from acetate formation. In contrast to the well characterized SCSs no information on structure and mechanism of ACD-like enzymes in archaea and bacteria is available so far. Despite similarities between ACDs and SCSs there are several differences in the amino acid sequences as well as the domain architecture of these proteins. Further, most bacterial ACD homologs contain an additional acetyltransferase domain of unknown function, fn this project the unknown bacterial ACD homologs will be characterized to identify their putative physiological function(s). Further, reaction mechanism of ACDs and their structure will be analysed in comparison to SCS. The studies will extend our knowledge of an unusual protein family in central archaeal and bacterial metabolism.