De novo proteins designed to study aromatic side-chain redox chemistry

摘要

Proteins play key roles in essentially all biological processes. The foundation for this remarkable functional diversity is provided by the structural and chemical properties of twenty different amino acids. In recent years, a specific feature of amino-acid functionality has moved into focus as four residues -- tyrosine, tryptophan, cysteine and glycine -- have been shown to form catalytically active, one-electron oxidized radicals (Stubbe and van der Donk, 1998). The family of proteins in which side-chain redox chemistry forms a principal mechanistic theme catalyzes a number of fundamental reactions in biology. The ribonucleotide reductase enzymes, for example, utilize three, if not all four, of the known redox-active side chains in the conversion of ribonucleotides to deoxyribonucleotides in all living organisms (Stubbe and van der Donk, 1998). The aromatic side-chain redox cofactors have been implicated in DNA repair (Aubert et al., 2000), lignin degradation (Whittaker et al.,1999), and they serve as redox mediators in several heme peroxidases (Stubbe and van der Donk, 1998, Ivancich et al., 1999). In addition, studies performed in the Babcock laboratory have shown that redox-active tyrosines are integral to the catalytic cycles of photosystem II (PSII) and cytochrome c oxidase (CcO), the two key enzymes involved in the major water to dioxygen and dioxygen to water redox cycle in Nature. Thus, in the end of the 80's Babcock, Barry, Debus and coworkers showed that a redox-active tyrosine links the photochemistry at the PSII reaction center with the water-splitting chemistry at the (Mn)_4 cluster (Babcock et al., 1989). In 1995, Babcock et al. proposed that this tyrosine operates as a H-atom transfer cofactor in the catalyticcycle of PSII. The H-atom abstraction model for PSII has been discussed and developed in a series of articles (see Tommos and Babcock, 2000, Hoganson and Babcock, 2000 and references therein). More recently, the MSU group showed that the histidine cross-linked tyrosine located at the active site of CcO is oxidized in one of the key intermediate state of the catalytic cycle. In their mechanistic model for dioxygen reduction in respiration, the cross-linked tyrosine serves as a H-atom donor during the oxygen-oxygen bond-breaking step (Proshlyakov et al., 2000).