Crystal structure of thermally stable homodimeric cytochrome c$^\prime$-$\beta$ from Thermus thermophilus
Acta Crystallographica Section F, 78 (6):
217--225 (June 2022)DOI: 10.1107/S2053230X22005088
Abstract
Cytochrome c$^\prime$-$\beta$ is a heme protein that belongs to the cytochrome P460 family and consists of homodimeric subunits with a predominantly antiparallel $\beta$-sheet fold. Here, the crystal structure of cytochrome c$^\prime$-$\beta$ from the thermophilic Thermus thermophilus (TTCP-$\beta$) is reported at 1.74\AA resolution. TTCP-$\beta$ has a typical antiparallel $\beta$-sheet fold similar to that of cytochrome c$^\prime$-$\beta$ from the moderately thermophilic Methylococcus capsulatus (MCCP-$\beta$). The phenylalanine cap structure around the distal side of the heme is also similar in TTCP-$\beta$ and MCCP-$\beta$, indicating that both proteins similarly bind nitric oxide and carbon monoxide, as observed spectroscopically. Notably, TTCP-$\beta$ exhibits a denaturation temperature of 117$^\circ$C, which is higher than that of MCCP-$\beta$. Mutational analysis reveals that the increased homodimeric interface area of TTCP-$\beta$ contributes to its high thermal stability. Furthermore, 14 proline residues, which are mostly located in the TTCP-$\beta$ loop regions, possibly contribute to the rigid loop structure compared with MCCP-$\beta$, which has only six proline residues. These findings, together with those from phylogenetic analysis, suggest that the structures of Thermus cytochromes c$^\prime$-$\beta$, including TTCP-$\beta$, are optimized for function under the high-temperature conditions in which the source organisms live.