Substrate binding location

Although FeMo-cofactor is clearly knpHcated in substrate reduction cataly2ed by the Mo-nitrogenase, efforts to reduce substrates using the isolated FeMo-cofactor have been mosdy equivocal. Thus the FeMo-cofactor s polypeptide environment must play a critical role in substrate binding and reduction. Also, the different spectroscopic features of protein-bound vs isolated FeMo-cofactor clearly indicate a role for the polypeptide in electronically fine-tuning the substrate-reduction site. Site-directed amino acid substitution studies have been used to probe the possible effects of FeMo-cofactor s polypeptide environment on substrate reduction (163—169). Catalytic and spectroscopic consequences of such substitutions should provide information concerning the specific functions of individual amino acids located within the FeMo-cofactor environment (95,122,149). 

Ji X, Tordova M, O Donnell R, Parsons JF, Hayden JB, Gilliland GL, et al. Structnre and function of the xenobiotic substrate-binding site and location of a potential non-substrate-binding site in a class pi glutathione -transferase. Biochemistry 1997 36 9690-702. 

Despite the wide diversity of enzyme structures, most enzyme activity follows a general mechanism that has several reversible steps. In the first step, a reactant molecule known as a substrate (S) binds to a specific location on the enzyme (E), usually a groove or a pocket on the surface of the protein E + S ES The substrate binds to the active site through intermolecular interactions that usually include significant amounts of hydrogen bonding. 

Both enzymes belong to the family of a,p-hydrolases." The active site of MeHNL is located inside the protein and connected to the outside through a small channel, which is covered by the bulky amino acid tryptophane 128." It was possible to obtain the crystal structure of the complex with the natural substrate acetone cyanohydrin with the mutant SerSOAla of MeHNL. This complex allowed the determination of the mode of substrate binding in the active site." A summary of 3D structures of known HNLs was published recently." " ... 

Kornilova, A.Y., Bihel, F., Das, C., and Wolfe, M.S. (2005) The initial substrate-binding site of y-sccrctasc is located on presenilin near the active site. Proc. Natl Acad. Sci. USA 102, 3230-3235. 

Dodd, J. R. and Christie, D. L. (2001) Cysteine 144 in the third transmembrane domain of the creatine transporter is located close to a substrate-binding site.. /. Biol. Chem. 276, 46983-46988. 

Almost all enzymes are proteins. They provide templates whereby reactants (substrates) can bind and are favorably oriented to react and generate the products. The locations where the substrates bind are known as active sites. Because of the specific 3D structures of the active sites, the functions of enzymes are specific that is, each particular type of enzyme catalyzes specific biochemical reactions. Enzymes speed up reactions, but they are not consumed and do not become part of the products. Enzymes are grouped into six functional classes by the International Union of Biochemists (Table 2.2). 

Enzymes may exist as simple monomers, or as homo- or heterodimers, or as multimers. In multimeric enzymes, each component monomer may possess a catalytic site alternatively, the catalytic site may be located at the interface between two or more monomers, or only one monomer of a heteromultimer may possess an active site. It is not uncommon in dimeric or multimeric enzymes containing two or more active sites for some degree of cooperativity to exist between the sites, with respect to the substrate binding or substrate turnover number (Monod et ah, 1965). 

The spectra from the NDO-naphthalene complex also revealed a second binding conformation (denoted as B), in which the substrate is located -0.5 A from the Fe atom. (From Yang et ah, 2003)... 

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