Oxygen enzymic catalysis

The chemical reaction catalyzed by triosephosphate isomerase (TIM) was the first application of the QM-MM method in CHARMM to the smdy of enzyme catalysis [26]. The study calculated an energy pathway for the reaction in the enzyme and decomposed the energetics into specific contributions from each of the residues of the enzyme. TIM catalyzes the interconversion of dihydroxyacetone phosphate (DHAP) and D-glyceraldehyde 3-phosphate (GAP) as part of the glycolytic pathway. Extensive experimental studies have been performed on TIM, and it has been proposed that Glu-165 acts as a base for deprotonation of DHAP and that His-95 acts as an acid to protonate the carbonyl oxygen of DHAP, forming an enediolate (see Fig. 3) [58]. 

Enzyme catalysis. An enzyme in the potato is catalyzing the decomposition of a hydrogen peroxide solution, as shown by the bubbles of oxygen. 

Tapia, O., Andres, J. and Safont, V. S. Enzyme catalysis and transition structures in vacuo. Transition structures for the enolization, carboxylation and oxygenation reactions in ribulose- 1,5-bisphosphate carboxylase/oxygenase enzyme (Rubisco), J.Chem.Soc.Faraday Trans., 90 (1994), 2365-2374... 

Although quite reliable empirical rules exist for the enantioselectivity of hydrolases for secondary alcohols (see Section 4.2.1.2), such rules are not as developed for primary alcohols, partly because many hydrolases often show low enantioselectivity. With some exceptions, lipases from Pseudomonas sp. and porcine pancreas lipase (PPL) often display sufficient selectivity for practical use. The model described in Figure 4.3 has been developed for Pseudomonas cepacia lipase (reclassified as Burkholderia cepacia), and, provided that no oxygen is attached to the stereogenic center, it works well for this lipase in many cases [41]. However, as soon as primary alcohols are resolved by enzyme catalysis, independent proof of configuration for a previously unknown product is recommended. 

Electron tunneling may also be of significance for redox catalysis, including enzyme catalysis. In particular it may turn out to be a tool for carrying out catalytic reactions via multi-electron paths. For instance, according to the data of ref. 11, the two-electron reduction of molecular oxygen to hyd-... 

Hashimoto S, Tatsuno Y, Kitagawa T (1986) Resonance Raman evidence for oxygen exchange between the FeIV=0 Heme and bulk water during enzymic catalysis of horseradish peroxidase and its relation with the heme-linked ionization. Proc Natl Acad Sci USA 83 2417-2421... 

The possible role of oxygen atom transfer in molybdenum enzyme catalysis was recognized in the early 1970s (190-194). In the ensuing years, a wealth of chemistry has established molybdenum as the premier exponent of such reactions (7, 195). Importantly, related dioxo-Mo(VI) and oxo-Mo(IV) complexes are interconverted by oxygen atom transfer reactions (Eq. (13)). These reactions are effected by reductants (X) such as tertiary alkyl and aryl compounds of the group 15 elements (especially phosphines) and oxidants (XO) such as S- and N-oxides. In many cases, however, the Mo(VI) and Mo(IV) compounds participate in a comproportionation reaction yielding dinuclear Mo(V) complexes (Eq. (15)). 

Cytochrome P450 Monooxygenases, Chemistry of Enzyme Catalysis, Roles of Structural Dynamics in Enzyme Catalysis, Chemical Strategies for NAD+ Dependent Enzymes, Chemistry of Oxygen-Activating Enzymes, Chemistry of Transient State Enzyme Kinetics Flavoenzymes, Chemistry of... 

A similar chelation of metal to enzyme-bound substrate may also contribute to enzyme catalysis of proton transfer at carbon. For example, X-ray crystallographic analysis of complexes bet veen 3-keto-L-gulonate 6-phosphate decarboxylase and analogs of the 1,2-enediolate reaction intermediate provide evidence that the essential magnesium dication is stabilized by coordination to both the C-2 oxygen and the nonreacting C-3 hydroxy of the reaction intermediate [88]. 

The pivotal role of different protonation pathways to boimd oxygen derivatives in bioinorganic chemistry has been shown in various mechanistic studies The same concept has also been considered for the case of catalysis involving HjOj, in which the correlation between the ability to efficiently deliver the proton to the metal-bound hydroperoxide anion, and the efficiency of catalytic decomposition of hydrogen peroxide has been noted. In heme-enzyme catalysis, however, the ability to supply the necessary one or two protons for the generation of the proper active intermediate from the iron-bound peroxide/ hydroperoxide anion state is an essential and crucial aspect of the detailed structural fine-tuning provided by the enz3me active center. 

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