The reactivity of haemoproteins and cytochromes

Abstract

The nature and control of electron-transport mechanisms in multi-enzyme sequences of cytochromes is considered. Four levels of electron-transfer activity involving cytochromes of types c or f have been identified. The 1st 2 processes are observed in photosynthetic bacterica, the latter pair in mitochondria. The 1st process is a temperature-insensitive reaction observed in Chromatium with a half-time of 2 msec. at 34[degree] K, which would probably occur almost as rapidly at 0[degree]K. One mechanism suggested for this reaction involves electron conduction from the haem of cytochrome c via its linkages to the periphery of the protein and eventually to the electron acceptor, chlorophyll, by means of a quantum-mechanical tunnelling of electrons through the barrier between the two. This reaction is, most probably, an electron transfer involving no thermal agitation, bond-breading or conformation change. A 2nd type of electron transport is observed in photosynthetic systems at room temperature and is much more rapid, with half-times of about 2 usec, for cytochrome c in Chromatium or 14 usec. for cytochrome f in Porphyridium. These are the fastest biological reactions yet directly recorded. They require thermal agitation and may or may not be accompanied by electron tunnelling. It is possible that this type of electron-transport reaction is too rapid to permit energy conservation, and serves only to transfer the electron-accepting site from chlorophyll to a more convenient location for thermal reactions in the photosynthetic structure. The 3rd, much slower, reaction is observed in electron transport between cytochromes of the respiratory chain in mitochondria. Here thermal agitations are necessary, not only to deliver the electron to the acceptor, but also to allow the acceptor to receive it from the donor. This reaction has a half-time of 1.8 msec. at 33[degree]. The 4th reaction is observed in the steady state of electron transport in the respiratory chain, and proceeds at rates as low as 0.5 sec.-1 in the controlled state 4. Although the cytochrome turnover numbers are very small compared with their oxidation rates, the cytochromes are appreciably reduced in the resting state of electron flow (state 4) and may become even more reduced in the active state (state 3). Thus attention is called to special factors controlling electron transport in reactions involved in energy conservation. Five mechanisms for the control of electron transport in energy-conserving systems are considered: 1st, by chemical modification of the cytochrome, for example by ligand substitution; 2nd, by a protein conformation change that decreases the activity of the cytochrome; 3rd, by an increase in the width or height of an electron-tunnelling barrier; 4th, by an increase in the rigidity of the protein structure that might inhibit electron flow in a manner similar to that by which it inhibits the binding of small ligands in ferrimyoglobin; 5th, by a generalized structural rearrangement of the components of the mitochondrial electron-transport chain. Though no one of these mechanisms seems, in itself, adequate to explain the experimental evidence for control of electron flow, 2 combinations of these possibilities are of great interest: a ligand interaction that alters protein flexibility, or a gross structural change of the mitochondrial membrane that alters protein mobility or tunnelling barrier width.