Stabilization of Pseudomonas aeruginosa Cytochromec 551 by Systematic Amino Acid Substitutions Based on the Structure of Thermophilic Hydrogenobacter thermophilus Cytochrome c 552

Abstract

A heterologous overexpression system for mesophilic Pseudomonas aeruginosa holocytochrome c 551 (PA c 551 ) was established using Escherichia coli as a host organism. Amino acid residues were systematically substituted in three regions of PA c 551 with the corresponding residues from thermophilic Hydrogenobacter thermophilus cytochrome c 552 (HT c 552 ), which has similar main chain folding to PA c 551 , but is more stable to heat. Thermodynamic properties of PA c 551 with one of three single mutations (Phe-7 to Ala, Phe-34 to Tyr, or Val-78 to Ile) showed that these mutants had increased thermostability compared with that of the wild-type. Ala-7 and Ile-78 may contribute to the thermostability by tighter hydrophobic packing, which is indicated by the three dimensional structure comparison of PA c 551 with HT c 552 . In the Phe-34 to Tyr mutant, the hydroxyl group of the Tyr residue and the guanidyl base of Arg-47 formed a hydrogen bond, which did not exist between the corresponding residues in HT c 552 . We also found that stability of mutant proteins to denaturation by guanidine hydrochloride correlated with that against the thermal denaturation. These results and others described here suggest that significant stabilization of PA c 551 can be achieved through a few amino acid substitutions determined by molecular modeling with reference to the structure of HT c 552 . The higher stability of HT c 552 may in part be attributed to some of these substitutions.

The Cytochrome c Fold Can Be Attained from a Compact Apo State by Occupancy of a Nascent Heme Binding Site

“…The stability of mitochondrial cytochrome c is, however, lower with a denaturation temperature (T m ) of 67°C for the oxidized state (31). Comparisons have been made of the structures and sequences of homologous proteins from thermophiles and mesophiles (32) including a comparative study of cytochrome c 552 from H. thermophilus and the homologous cytochrome c 551 from mesophilic Pseudomonas aeruginosa (33,34). One feature identified that may be important in promoting the thermal stability of H. thermophilus cytochrome c 552 is the high density of hydrophobic side chains in the core of this protein (34).…”

Correlation between the Stability and Redox Potential of Three Homologous Cytochromescfrom Two Thermophiles and One Mesophile

“…The HT c 552 , PH c 552 , and PA c 551 wild-type proteins, and the reciprocal variants, HT c 552 I78V, PH c 552 V78I, and PA c 551 V78I, were prepared by the method previously described. 6,9,10) We adopted the residue numbering system used for PA c 551 for clarity. The proteins were overexpressed with co-transformed pEC86 carrying the ccmABCDEFGH genes in Escherichia coli JCB387 cells and isolated from periplasmic extracts.…”

Effects of Cysteine Introduction into Three Homologous Cytochromesc

“…The HT c 552 , PH c 552 , and PA c 551 wild-type proteins, and the variants, HT c 552 S78C, PH c 552 T77C, and PA c 551 S80C, were prepared by a method previously described, 6,8,9) with minor modifications. The proteins were overexpressed with co-transformed pEC86 carrying the ccmABCDEFGH genes in Escherichia coli JCB387 cells and isolated from the periplasmic extracts.…”

“…8) Data points were corrected for the slope of the baselines for the native and denatured forms, and normalized to calculate the fraction of protein denatured. The fraction denatured was plotted as a function of temperature ( Fig.…”