Substrate enzyme interactions, electronic

It thus appears that quantum chemical methods can successfully predict and characterize electronic mechanisms in substrate - enzyme interactions on the basis of the molecular reactivities of the separated entities, and from results of simulated interactions between molecular models - as shown by the following conclusions ... 

In spite of the close structural relationship of the molybdenum hydroxylases, including a tendency for hydrophobic substrate/enzyme interaction, there is a very significant difference in the substrate specificity of the two enzymes. Not only is there considerable variation in the affinities for substrates and inhibitors, but there is often a difference in the position of oxidative attack. As both enzymes catalyse apparently similar nucleophilic reactions, this difference cannot be explained solely from electronic considerations and is probably due, to a great extent, to the differential response of each enzyme to steric factors. 

Extensive studies have established that the catalytic cycle for the reduction of hydroperoxides by horseradish peroxidase is the one depicted in Figure 6 (38). The resting enzyme interacts with the peroxide to form an enzyme-substrate complex that decomposes to alcohol and an iron-oxo complex that is two oxidizing equivalents above the resting state of the enzyme. For catalytic turnover to occur the iron-oxo complex must be reduced. The two electrons are furnished by reducing substrates either by electron transfer from substrate to enzyme or by oxygen transfer from enzyme to substrate. Substrate oxidation by the iron-oxo complex supports continuous hydroperoxide reduction. When either reducing substrate or hydroperoxide is exhausted, the catalytic cycle stops. 

Second, formation of weak bonds between substrate and enzyme also results in desolvation of the substrate. Enzyme-substrate interactions replace most or all of the hydrogen bonds between the substrate and water. Third, binding energy involving weak interactions formed only in the reaction transition state helps to compensate thermodynamically for any distortion, primarily electron redistribution, that the substrate must undergo to react. 

M(VI) and M(IV) oxidation states. The M(V) state is generated by a one-electron reduction of the M(VI) state, or the one-electron oxidation of the M(IV) state, and occurs during the catalytic cycle—en route to the regeneration of the catalytically active state. Spectroscopic studies of the Mo—MPT enzymes, notably electron spin resonance (EPR) investigations of the Mo(V) state, have clearly demonstrated that the substrate interacts directly with the metal center (37). The first structural characterization of a substrate-bound complex was achieved for the DMSOR from Rhodobacter capsulatus DMS was added to the as-isolated enzyme to generate a complex with DMSO that was O-bound to the molybdenum (43). 

The function of peroxidase enzymes is the activation of HOOH to provide two oxidizing equivalents for the oxidation of a variety of substrates. The interaction of horseradish peroxidase (HRP, an iron(in) heme that has a proximal imidazole) with HOOH results in the formation of a green reactive intermediate known as Compound I. It is reduced by one electron to give a red reactive intermediate, Compound II. Both of these intermediates contain a single oxygen atom from HOOH, and Compound I is two oxidizing equivalents above the iron(III)-heme state with a magnetic moment equivalent to three unpaired electrons (S = 3/2). A recent extended X-ray-absorption fine-structure (EXAFS) study summarizes the physical data in support of (por+-)Fe =0 as a... 

In summary, vibrational spectroscopic studies have shown that in the enzyme catalyzed hydride transfer reaction, the substrate C=0 or C=N bond in the Michaelis complex may first be activated in two different ways for the subsequent hydride transfer. For a substrate that contains a C=0 bond to be reduced by the enzyme, strong electron withdrawing interaction due to hydrogen bonding (in LDH) or electrostatic interaction (in LADH) can polarize this bond to reduce the electron cloud near the carbonyl carbon to facilitate the hydride transfer. This is consistent with theoretical studies that one of the driving forces for the hydride transfer is the large... 

Type I substrates such as hexobarbital, ethylmorphine and testosterone which cause loss of absorbance at 420nm with a concomitant increase at 385nm, and Type II substrates such as aniline, pyridine and acetanilide which cause an increase in absorbance at 430nm and a decrease at 400nm. Such alterations in the heme chromophore reflect interaction at or near the active site of the enzyme. Type II substrates have nonbonding electron pairs which are thought to interact with the heme iron. Three types of EPR spectra have been seen for oxidized cytochrome P-450, two low spin forms and one high spin form. One of the low spin forms is associated with substrate-free oxidized P-450, while the other is observed... 

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