[Mechanism of action of cAMP-dependent protein kinase. II. Solvolysis by additional nucleophilic agents]

The reaction of the solvolysis of the phosphointermediate, formed in the course of the phosphotransferase reaction was studied using a number of non-specific nucleophilic agents. The methods of identification of the solvolysis products and determination of kinetic parameters were developed. The estimated dependence of the reaction of the reactivity of the nucleophilic agents versus their basicity allowed some conclusions about the transition state structure in the cases of enzyme's interaction with nucleophilic agents and protein substrate. The energy contribution of the specific interaction of the enzyme with histone H1 was evaluated.

[Study by the spin-label method of relaxation properties of protein kinase, its subunits and the catalytic subunit--histone H1 complex]

Using the spin label method, the rotational relaxation in solution of adenosine 3',5'-monophosphate-dependent protein kinase and its subunits as well as the complexes of the enzyme with the substrate, histone H1, was studied. The rotational correlation time of the spin labeled macromolecules was measured on the basis of the quantitative estimation of the label mobility in relation to the protein globule. The holoenzyme molecule was found to be a rigid sphere. Whereas the complex of the globular catalytic subunit of the enzyme with a specific protein substrate, the spin labeled histone H1, appeared a flexible formation. The relaxation properties of the histone H1 molecule selectively labeled by the spin label in its globular part were investigated.

Studies on the mechanism of action of the histone kinase dependent on adenosine 3',5'-monophosphate. Interaction of ATP with the catalytic subunit of the pig-brain enzyme: application of the quenched-flow technique

A significant release of inorganic phosphate from ATP in the presence of the cAMP-dependent pig-brain histone kinase was detected. The high degree of homogeneity of the enzyme preparations used, identity of Michaelis constants (Km for ATP = 12 microM), the close values of cAMP activation constants (48 nM and 51 nM) for the phosphotransferase and ATPase activities, respectively, are all evidence that ATP decomposition is catalysed by the histone kinase under study. The ATPase activity observed supports the ping-pong bi-bi mechanism established earlier for the phosphotransferase reaction and can be regarded as due to decomposition of the phosphoryl enzyme. The transient and steady-state phases of the ATP hydrolysis were studied. The simplest reaction pathway can be described in terms of a three-step mechanism. The close values of the rate constant for the elementary stages of the ATPase reaction obtained in the nucleophile competition study and by computer simulation of the quenched-flow kinetics give further support for the mechanism proposed. The phosphoryl enzyme decomposition was shown to be a rate-limiting step under the experimental conditions used (pH 7.8-8.0).

Determination of binding parameters of cyclic AMP and its analogs to cyclic AMP-dependent protein kinase by the fluorescent probe method

The method for determination of dissociation constants for cyclic AMP and its analogs bound to cyclic AMP-dependent protein kinase from pig brain is described. The technique for measuring the binding parameters of the ligands is based on the changes in the fluorescent spectrum of etheno cyclic AMP once it is bound to protein kinase. The dissociation constants for a number of nonfluorescent cyclic AMP analogs were determined in the competitive displacement of etheno cyclic AMP by these analogs. The number of cyclic AMP-binding sites in the pig brain protein kinase was found to be 2.2; no cooperativity was observed upon binding. The holoenzyme complex (Mr = 180,000) of the protein kinase under study was established to have the stoichiometry of R2C2 type under native conditions.

[Mechanism of action of cyclic AMP-dependent histone kinase. Substrate specificity of the catalytic enzyme unit]

Interaction of several nucleotide derivates with homogenous catalytic subunit of cyclo-AMP-dependent histone kinase from pig brain is studied. Inhibition constants of these compounds are calculated, and the affinity of inhibitors to the enzyme active site is evaluated. The nature of heterocyclic base is found to be the main contribution into binding with substrate. The enzyme specificity with respect to a number of bivalent metal ions is studied, and Mg2+ is demonstrated to be the only efficient enzyme activator. It is shown by means of stationary kinetics that histone kinase-catalysed phosphotransferase reaction has a "ping-pong"-like mechanism.

Studies on the mechanism of action of histone kinase dependent on adenosine 3':5'-monophosphate. Evidence for involvement of histidine and lysine residues in the phosphotransferase reaction

The reaction of the phosphate residue transfer catalysed by histone kinase dependent on adenosine 3':5'-monophosphate (cyclic AMP) was studied. The phosphotransferase reaction was shown to obey the mechanism of ping-pong bi-bi type. After incubation of the catalytic subunit of histone kinase with [gamma-32P]ATP the incorporation of one mole of [32P]phosphage per mole of protein was observed. The tryptic [32P]phosphohistidine-containing peptide was isolated and its N-terminus and amino acid composition were determined. The 2',3'-dialdehyde derivative of ATP (oATP) was used as the affinity label for the catalytic subunit of cyclic-AMP-dependent histone kinase. The inhibitor formed an alidmine bond with epsilon-amino group of the lysine residue of the active site and was irreversibly bound to the enzyme after reduction by sodium borohydride with concurrent irreversible inactivation of the enzyme. After inactivation, about one mole of 14C-labelled inhibitor was incorporated per mole of the enzyme. ATP effectively protected the catalytic subunit of histone kinase against inactivation by oATP. Tryptic digestion of the enzyme-inhibitor complex led to the isolation of the 14C-labelled peptide of the active site of histone kinase. Basing on these results, the role of histidine and lysine residues in the active site of the catalytic subunit of histone kinase was suggested.

[Sulfhydryl groups of cyclic AMP-dependent histone kinase from pig brain]

A preparative method of isolation of the histone kinase regulatory subunit has been developed. The total number of cysteine residues in the enzyme (13 residues) and in each of its subunits (3 residues in the catalytic subunit and 10 residues in the regulatory one), has been determined. Two disulfide bridges have been found in the regulatory subunits. The relative availability of the sulfhydryl groups with respect to different modifying agents has been studied. It has been demonstrated that SH-groups blocking does not change essentially the enzyme activity.