Organic redox cofactors.

721 REDOX-ACTIVE COFACTORS, including quinones and flavins, are important components in biological systems. Together with the apoprotein, they perform complex redox chemistry, provide an important link in signal transduction, and participate in crucial electron transfer pathways, aspects that are often inefficiently carried out by the proteins themselves. These apoprotein–cofactor assemblies are essential in all aspects of cellular function, making them important targets for chemical and biochemical investigations. Although there is enormous diversity in the processes mediated by organic cofactors, the fact that the same redox unit is utilized to perform a multiplicity of functions provides common ground for researchers from a wide variety of fields. It is this multiand interdisciplinary approach that we have focused on for this issue, bringing together chemists, biochemists, and molecular biologists that explore key issues of cofactor-mediated processes. Unlike many metal centers, the chemistry of organic cofactors can be studied outside of the apoenzyme environment using synthetic and computational model systems as powerful tools for the understanding of their more complex enzymatic prototypes. In a broader sense, redox proteins (and the systems that model them) provide a unique milieu for the study of enzymatic processes. The cofactor provides an “active site” with relatively few structural differences relative to most other enzymatic systems. This greatly simplifies the catalytic event, allowing the role of individual effects to be explored using both molecular and supramolecular methods. The relatively small size of organic cofactors allows for the application of high-level computational methods in the modeling of biological systems. In this issue, a communication by Rizzo describes the use of ab initio computational methods to explore the structure of an FADH2 analog. Also, O’Malley describes the correlation of density functional calculations and electron paramagnetic spectroscopic studies of quinone cofactors. In the area of cofactor chemistry, Carson, Tam-Chang, and Beck illustrate the use of surface-immobilized flavins to study microenvironmental effects on the redox properties of flavin cofactors. Supramolecular chemistry provides an additional tool for the modeling of cofactor-mediated processes. Reviews by Yano as well as Fukuzumi and Itoh describe applications of this approach in the modeling of flavoenzymes and quinoenzymes, respectively. Spectroscopy provides a bridge between chemistry and biochemistry, allowing information transfer between the simpler model systems and their more complex biological prototypes. In this issue, Stanley provides an up-to-date review of the application of a number of spectroscopic techniques to the study of flavins and flavoenzymes. The structural attributes of cofactor-containing proteins make them particularly good candidates for in-depth studies of structure and