Catalysis in biological systems

Traditionally when considering catalysis in biological systems primary emphasis has been placed on the enzymes—very high-molecular-weight proteins. However, recently there has been growing interest in the role of the transition metals, particularly as found in coenzymes such as BI2, in biochemical reactions. 

Cammack, R. Catalysis by Nickel in Biological Systems Marcel Dekker New York, 1993. 

In the course of evolution, the importance of particular metal ions in biological systems has ebbed and flowed, as a function of environmental conditions. Before the arrival of photosynthesis, when there was no oxygen, elements like Fe and Ni were extremely important, whereas, for example, Cu was virtually inaccessible for reasons of solubility. With the arrival of an oxidizing environment, Ni virtually disappeared from the equation, Cu became bioavailable, and Fe, although it was now insoluble and poorly available, had proved of such fundamental importance in biological catalysis that specific systems were developed for its uptake from the environment, such that it continues to play a key role in life as we know it today. 

Table 1.6 Metals in Biological Systems Enzyme Catalysis...

Cammack, R. and van Vliet, P. (1999) Catalysis by Nickel in Biological Systems. In J. Reedijk and E. Bouwman (eds). Bioinorganic Catalysis, 2nd edn. New York Marcel Dekker, pp. 231-68. 

Other reviews on proton transfer deal with the following topics hydrogen bonding and H+ transfer " , general acid/base catalysis " and proton transfers in biological systems involving proteins . 

The problems above mostly involve homogeneous oxidations. Another objective of this symposium was to find out how similar are the mechanisms and reactions in homogeneous oxidations to those in heterogeneous catalysis and biological systems. So far it seems that they are not very similar because neither ordinarily involves free radicals. However, the methods used to study biological oxidations have much in common with those used by physical-organic chemists in homogeneous oxidations. 

Ribonucleotide reductase is notable in that its reaction mechanism provides the best-characterized example of the involvement of free radicals in biochemical transformations, once thought to be rare in biological systems. The enzyme in E. coli and most eukaryotes is a dimer, with subunits designated R1 and R2 (Fig. 22-40). The R1 subunit contains two lands of regulatory sites, as described below. The two active sites of the enzyme are formed at the interface between the R1 and R2 subunits. At each active site, R1 contributes two sulfhydryl groups required for activity and R2 contributes a stable tyrosyl radical. The R2 subunit also has a binuclear iron (Fe3+) cofactor that helps generate and stabilize the tyrosyl radicals (Fig. 22-40). The tyrosyl radical is too far from the active site to interact directly with the site, but it generates another radical at the active site that functions in catalysis. 

Iron as a cofactor in catalysis is receiving increasing attention. The most common oxidation states of iron are Fe2+ and Fe3+. Iron complexes are nearly all octahedral, and practically all are paramagnetic (as a result of unpaired electrons in the 3d orbital). The most common form of iron in biological systems is heme. Heme groups (Fe2+) and hema-tin (Fe3+) most frequently involve a complex with protoporphyrin IX (fig. 10.19). They are the coenzymes (prosthetic... 

RNA catalysis has been proposed for use in preparing combinatorial libraries of organic structures for drug discovery [39]. As we learn more about the scope, reactivity, and specificity of RNA as a catalyst for organic reactions, it should be possible to use RNA to create new chemical diversity that parallels that found in biological systems, where proteins are the catalysts in the formation of natural products. 

This book attempts to describe alternative approaches to ligand reactivity involving normal co-ordination complexes as opposed to organometallic compounds. In part, a justification for this view comes from a study of natural systems. With very few exceptions, organometallic compounds are not involved in biological systems it is equally true that numerous enzymes bind or require metal ions that are essential for their activity. If enzymes can utilise metal ions to perform complex and demanding organic chemical reactions in aqueous, aerobic conditions at ambient temperature and pressure, it would seem to be worthwhile to ask the question whether this is a better approach to catalysis. 

A simplified picture of catalysis in biological, interfacial and chemical systems. ... 

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