[Effect of the structure of the prosthetic group on the catalytic properties of prostaglandin H synthetase]. / Izuchenie vliianiia struktury prosteticheskoi gruppy na kataliticheskie svoistva prostaglandin-H-sintetazy.

The effect of vinyl groups of protohemin IX on its cofactor properties with respect to prostaglandin H synthetase has been studied. It was shown that substitution of ethyl groups or a hydrogen for vinyl groups affects neither binding of the prosthetic group to the apoenzyme nor catalytic properties of holo-prostaglandin H synthetase. Replacement of vinyl groups with bulkier substituents (hydroxyethyl or acetyl groups) decreases holoenzyme stability and catalytic activity. By comparison of the cofactor properties of protoporphyrin and hematoporphyrin macrocycles with different central ions (Fe3+, Mn2+, 2H+ in the case of protoporphyrin, and Fe3+, Mg2+, Cd2+ and Cu2+ in the case of hematoporphyrin), the presence of Fe3+ ions was shown to be mandatory for prostaglandin H synthetase activity. It was demonstrated that the cofactor structure modifications do not affect the holo-prostaglandin H synthetase inactivation rate constant in a reaction.

[Effect of the modification of carboxylic groups in protohemin IX on the properties of endoperoxide prostaglandin synthetase]. / Vliianie modifikatsii karboksil'nykh grupp protogemina IX na svoistva éndoperoksidprostaglandinsintetazy.

The effect of protohemin IX and its modified analogs (monomethyl ester, dimethyl ester, as well as monoamides with Val-Phe-OCH3 or Leu-His-OCH3) has been examined on the activity of prostaglandin endoperoxide synthetase from sheep vesicular glands (PGH-synthetase, EC 1.14.99.1, isolated as apoenzyme). For holoenzymes having the above compounds as prosthetic groups, the dissociation constants, relative activities and the apparent inactivation constants in the course of the reaction have been determined. The effect of Tween 20 on the indicated parameters for holoenzymes with protohemin IX and its mono- and dimethyl esters has been studied. Modification of one of the two carboxylic groups of protohemin IX markedly increases the dissociation constant for the respective holoenzyme and virtually does not affect catalytic activity. Modification of both carboxylic groups of protohemin IX hinder the binding with the apoenzyme and strongly reduces the catalytic activity of the holoenzyme.