Puustinen A, Morgan J E, Verkhovsky M, Thomas J W, Gennis R B, Wikström M
Department of Medical Chemistry, University of Helsinki, Finland.
Biochemistry. 1992 Oct 27;31(42):10363-9. doi: 10.1021/bi00157a026.
Cytochrome o of Escherichia coli is able to incorporate two different structures of heme, either heme B (protoheme) or heme O, in its low-spin heme site. In contrast, the heme of the binuclear O2 reduction site is invariably heme O. Heme O is a newly discovered heme that is related to heme A, but with the formyl group of the latter replaced by methyl. Enzyme isolated from wild type E. coli has predominantly heme B in the low-spin site, whereas enzyme isolated from various overexpressing strains contains both types of enzyme in different proportions. In some strains, 70% of the enzyme has heme O in the low-spin site. Despite this variation in the structure of one of the prosthetic groups, the enzymatic activity and polypeptide composition of the enzyme remain virtually constant. EPR and activity data both indicate that heme B and heme O occupy the same low-spin heme site in the enzyme. With heme O in this site, the alpha-absorption band is narrower and further to the blue, and the Em,7 is lower, than when there is heme B in the site. In contrast to previous proposals, we show here that the enzyme does not exhibit significant spectral interactions between the hemes. The structural heterogeneity of the low-spin heme accounts for the variation in the optical spectra and redox properties of the enzyme as isolated from different strains of E. coli.
大肠杆菌的细胞色素 o 能够在其低自旋血红素位点结合两种不同结构的血红素，即血红素 B（原血红素）或血红素 O。相比之下，双核 O2 还原位点的血红素始终是血红素 O。血红素 O 是一种新发现的与血红素 A 相关的血红素，但后者的甲酰基被甲基取代。从野生型大肠杆菌中分离出的酶在低自旋位点主要含有血红素 B，而从各种过表达菌株中分离出的酶含有不同比例的两种类型的酶。在一些菌株中，70%的酶在低自旋位点含有血红素 O。尽管其中一种辅基的结构存在这种差异，但该酶的酶活性和多肽组成实际上保持不变。电子顺磁共振（EPR）和活性数据均表明，血红素 B 和血红素 O 在该酶中占据相同的低自旋血红素位点。当该位点存在血红素 O 时，α吸收带更窄且向蓝色方向移动更远，并且 Em,7 更低，而当该位点存在血红素 B 时则不然。与之前的提议相反，我们在此表明该酶在血红素之间不表现出明显的光谱相互作用。低自旋血红素的结构异质性解释了从不同大肠杆菌菌株中分离出的该酶的光谱和氧化还原性质的变化。
