Structure enzyme-anion complexes

The enzyme is a hexamer, actually a dimer of trimers made up of 291-residue polypeptide chains.28 Aceto-acetyl-CoA is a competitive inhibitor which binds into the active site and locates it. From the X-ray structure of the enzyme-inhibitor complex it can be deduced that the carboxylate group of E144 abstracts a proton from a water molecule to provide the hydroxyl ion that binds to the P position (Eq. 13-6, step a) and that the E164 carboxyl group donates a proton to the intermediate enolate anion in step b.28 The hydroxyl group... 

The ability of zinc in carbonic anhydrase to become five-coordinate is also confirmed by the structural studies on enzyme-inhibitor complexes discussed in Section 62.1.4.2.1. There is much evidence for the coordination of anionic inhibitors to the metal, while the competitive inhibitor imidazole gives a five-coordinate centre. Sulfonamides are powerful inhibitors which bind directly to the zinc and also interact with the protein. The sulfonamide acetazolamide has significant affinity for the apoenzyme. It is probable " that the first interaction between the enzyme and aromatic sulfonamides is a hydrophobic interaction between the aromatic ring and residues in the active site cavity, followed by ionization of the SO2NH2 group prior to complex formation. Sulfonamides only bind to the zinc and cobalt enzymes, i.e. the two metals that give an active enzyme. 

Generally, the reactivating efficacy of oximes depends on their reactivity and affinity for OPC-inhibited enzyme. Their reactivity is derived from the nucleic activity of oxime anion that is bound on the pyridinium ring (8). Oximes differ from each other by the position of the oxime group on the pyridinium ring only. The reactivity of all available oximes is almost the same because their basic structure is very similar (8). The affinity of oximes for intact enzyme, characterized by dissociation constant of enzyme-reactivator complex (Kdls), and for nerve agent-inhibited enzyme, characterized by dissociation constant of inhibited enzyme-reactivator complex (Kr), is determined by various physicochemical factors such as steric compatibility, electrostatic effects, hydrophobic interactions and by the shape and the size of the whole molecule as well as functional groups (22). 

The IR studies of Riepe and Wang 125) indicate that CO2 is not coordinated to zinc ion in the enzyme-C02 complex, and CO 2 does not perturb the spectrum of the Co(II) enzyme (72/). (2) Alkylation of His-63 with bromopyruvate 135) only brings about a 70 percent decease in the activity of human carbonic anhy-drase C. Therefore, the chemical integrity of His-63 is not crucial to catalytic activity. (3) It is unlikely that anion or sulfonamide binding could sufficiently perturb the pKa of His-63 to account for the ionization properties of these complexes, and it is not apparent from the X-ray structure of the Co(II) enzyme 41, 106) how the ionization of His-63 can account for the pH-dependence of its visible cobalt(II) spectrum. 

Apparent dissociation constants for nitrate complexes of nitrate reductase and of the two xanthine oxidase forms are in the range 4-20 mM. For nitrate reductase some evidence for a complex with azide was also obtained, but no other ions tested had significant effects on the spectra (Vincent and Bray, 1978). Clearly further work on the anion complexes is required. Ideally, one would like to see parameters for a range of anions all occupying the same enzyme site. From these, hopefully, some structural information might be deducible. 

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