Synopsis
The association of coenzyme A(CoASH) and glutathione(GSH) with the water-soluble polymers and their esterolytic reactivities were evaluated through the reaction with p-nitrophenyl acetate in the presence of cationic polymer micelles: partially laurylated poly(2ethyl-1-vinylimidazole) and poly(4-vinylpyridine). The polymer micelles with high laurylgroup content (more than 12 mol %) markedly accelerated the reaction a t very low concentrations of the polymer. Other polymers with no or small lauryl-group content only slightly enhanced the association and the reaction rate. From the rate-polymer concentration profiles, the association constants ( K ) and the rate constants for thiol coenzymes bound to the polymer (k&,und)
were determined: for polymers with more than 12 mol % lauryl-group content, kh,hund were 20-340 times larger than that observed in the absence of the polymer. The logarithm Of kb,hund was found to be correlated well with the polymer hydrophobicity, indicating that the hydrophobic environment of the polymer activated the bound thiol anions. On the other hand, the polymer hydrophobicity did not correlate with the association constant.
KC~ASH
= 1110-2270 M-l, K C ~H = 170-503M-', kb,bound a t pH 8.65 = 142-341M-' sec-'.
INTRODUCTION*
Most coenzymes are water soluble and exert high enzymatic reactivities only when they are bound to apoenzymes. It is known, for example, that the nucleophilic reactivity of glutathione, a biological thiol, is not very different from that of simple alkanethiols in aqueous media,lV2 and incomparably ineffective relative to that observed in uiuo. It is clear, therefore, that the microenvironments of apoenzymes must be responsible for the activation of bound coenzymes. It is said that the active sites of apoenzymes are situated in the hydrophobic r e g i ~n , ~, ~ but the essential role of the hydrophobic environment is not established.
Recently, it was shown that the nucleophilicity of some anionic nucleophiles (including thiol groups) bound to cationic polymer m i ~e l l e s , ~-~ as well as to the conventional cationic micelles,9-'6 is dramatically enhanced.
* Raw kinetic data (Figures 1M-4M) are stored in the microfiche repository of Biopolyrners.