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PubMed1988

Proteolysis of Paracoccus denitrificans cytochrome oxidase by trypsin and chymotrypsin.

Paracoccus oxidase containing only two subunits was subjected to proteolysis by trypsin and chymotrypsin. Both subunits of the purified enzyme were cleaved at only a few sites and enzymatic activity was not inhibited. The cleavage sites were identified by protein sequencing. Subunit I was cleaved near the amino-terminus and subunit II in the loop connecting the two predicted trans-membrane helices. In native membrane fragments, but not in intact spheroplasts, this loop was accessible to both proteases. These results provide experimental evidence for the folding of subunit II in the membrane.

Chymotrypsin↗
PubMed1996

Histidine 131, not histidine 43, of the Bradyrhizobium japonicum FixN protein is exposed towards the periplasm and essential for the function of the cbb3-type cytochrome oxidase.

In subunit I (FixN) of the Bradyrhizobium japonicum cbb3-type oxidase, only five instead of the normal six strictly conserved histidines (H) could be unambiguously assigned as the putative heme or copper ligands. The ambiguity concerned H43 or H131 as the presumptive N-terminal ligands of the low-spin heme B. We report here that a H43A replacement had a wild-type phenotype, whereas the H131A mutant was defective in oxidase function and subunit assembly or stability, suggesting that H131 serves as the N-terminal low-spin heme ligand. Topological studies revealed that H131 resides on the periplasmic side of helix 2, where one of the low-spin heme ligands is normally found in conventional heme-copper oxidases.

Bacterial Proteins↗