Molecular recognition in protein complexes involved in electron transfer.
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Introduction Electron transfer reactions involving protein complexes are important in many biological processes, including photosynthesis, respiration, nitrogen fixation and steroid synthesis. The interprotein electron transfer reaction occurs either by orbital overlap or through a number of chemical bonds in the polypeptide chain that act as pathways for the electrons to communicate with the two redox centres. Flavoproteins are important components of the electron transfer chains which participate in many processes in all kinds of organisms. They all have a flavin group tightly bound to the polypeptide chain, which gives the bright yellow colour characteristic of this type of proteins. In many flavoproteins the flavin group can accommodate the exchange of either one or two electrons during the redox reaction because the oxidized, semiquinone and fully reduced forms are stable. Free flavins are stable only in the fully oxidized and fully reduced states. Iron-containing proteins, such as cytochromes of the different classes and iron-sulphur proteins, are also present in these pathways. Although some of these proteins have more than one iron atom in the molecule, they only exchange one electron. Reactions in which reducing equivalents are transferred from (or to) pyridine nucleotide cofactors (typical two-electron transferring compounds) to iron proteins (which can accommodate only one electron) necessarily require the mediation of a flavoprotein to split the two-electron transferring reaction into two one-electron reactions. Transient complexes between flavoproteins, iron-sulphur proteins and cytochromes are, thus, the material of study of these type of reactions. Some of the basic questions concerning the mechanism of these reactions are under intense investigation by different groups and include the following: