Abstract
Two distinct class I (monoheme) c-type cytochromes from the hyperthermophilic bacterium Aquifex aeolicus were studied by biochemical and biophysical methods (i.e., optical and EPR spectroscopy, electrochemistry). The sequences of these two heme proteins (encoded by the cycB1 and cycB2 genes) are close to identical (85% identity in the common part of the protein) apart from the presence of an ...
Abstract
Two distinct class I (monoheme) c-type cytochromes from the hyperthermophilic bacterium Aquifex aeolicus were studied by biochemical and biophysical methods (i.e., optical and EPR spectroscopy, electrochemistry). The sequences of these two heme proteins (encoded by the cycB1 and cycB2 genes) are close to identical (85% identity in the common part of the protein) apart from the presence of an N-terminal stretch of 62 amino acid residues present only in the cycB1 gene. A soluble cytochrome was purified and identified by N-terminal sequencing as the cycB2 gene product. It showed an a-peak at 555 nm, an Em value of +220 mV, and electron paramagnetic resonance parameters of g(z) = 2.89, g(y) = 2.287, and g(x) = 1.52. A firmly membrane-bound cytochrome characterized by nearly identical properties was detected and attributed to the cycB1 gene product. The very high degree of homology of its N-terminal part to cytochrome C-553 from Heliobacterium gestii strongly suggests it to be anchored to the membrane via N-terminally attached lipid molecules. The two heme proteins were named cytochrome e(555)(s) (soluble) and cytochrome e(555)(m) (membranous). Electron paramagnetic resonance on partially ordered membrane multilayers suggests that the solvent-exposed heme domain of cytochrome c(555)(m) is flexible with respect to the membrane plane. Possible functional roles for both cytochromes are discussed.
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