Enzymes configuration

Seliger, H. H., and McElroy, W. D. (1964). The colors of firefly bioluminescence enzyme configuration and species specificity. Proc. Natl. Acad. Sci. USA 52 75-81. 

Probably all adenylyl cyclases are inhibited competitively by substrate analogs, which bind at the site and to the enzyme configuration with which cation-ATP binds (cf Fig. 4). One of the best competitive inhibitors is (3-L-2, 3 -dideoxy adenosine-5 -triphosphate ( 3-L-2, 3 -dd-5 -ATP Table 4) [4], which allowed the identification of the two metal sites within the catalytic active site (cf Fig. 4) [3]. This ligand has also been labeled with 32P in the (3-phosphate and is a useful ligand for reversible, binding displacement assays of adenylyl cyclases [4]. The two inhibitors, 2, 5 -dd-3 -ATP and 3-L-2, 3 -dd-5 -ATP, are comparably potent... 

P-site ligands inhibit adenylyl cyclases by a noncompetitive, dead-end- (post-transition-state) mechanism (cf. Fig. 6). Typically this is observed when reactions are conducted with Mn2+ or Mg2+ on forskolin- or hormone-activated adenylyl cyclases. However, under- some circumstances, uncompetitive inhibition has been noted. This is typically observed with enzyme that has been stably activated with GTPyS, with Mg2+ as cation. That this is the mechanism of P-site inhibition was most clearly demonstrated with expressed chimeric adenylyl cyclase studied by the reverse reaction. Under these conditions, inhibition by 2 -d-3 -AMP was competitive with cAMP. That is, the P-site is not a site per se, but rather an enzyme configuration and these ligands bind to the post-transition-state configuration from which product has left, but before the enzyme cycles to accept new substrate. Consequently, as post-transition-state inhibitors, P-site ligands are remarkably potent and specific inhibitors of adenylyl cyclases and have been used in many studies of tissue and cell function to suppress cAMP formation. 

Entry Enzyme Configuration/% ee of remaining ester Relative conversion... 

As far as Michaelis-Menten enzymes are concerned, i vs diagrams have been produced109 for various immobilized enzyme configurations. They can, therefore, be used provided that the geometry of the segregated regions is defined and that external resistances are taken into account. 

Some metal ions are known to be carcinogenic and this may be associated with the inducing of unfavourable enzyme configurations. The substitution of Mn for Mg + in the action of DNA polymerase is known to be mutagenic and to introduce errors in the fidelity of DNA synthesis. 

The mechanisms of enzyme catalytic actions are much more complex than those used to describe traditional catalysis, since enzyme configuration itself is strongly affected by the reaction environment and by the presence of specific substrates. 

This mechanism does not explain all of the observations made with this enzyme. The role of DPN in the exchange reactions remains obscure. It is always possible to consider that DPN is not concerned directly with exchange reactions, but is necessary for maintenance of enzyme configuration. Careful kinetic measurements have eliminated the possibility that traces of DPNH could be involved in a reversible oxidation. The role of sulfhydryl compounds is also not obvious. Glutathione, which activates and protects the enzyme, permits acyl exchange reactions after the enzyme has been inhibited by iodoacetate, yet inhibits the hydrolysis... 

Potassium phosphotungstate, generally abbreviated PTA, and uranyl acetate are used extensively as staining agents. Stains do not alter irreversibly enzyme configurations in aqueous solution since enzymatic activity is restored upon removal of stain by dialysis (Mel-lema et al., 1968). 

---