The interaction of phenylglyoxal with soluble and membrane-bound chloroplast coupling factor 1

Energy-dependent changes in the conformation of the chloroplast ATP synthase and its catalytic activity

Chloroplast ATP synthase changes its conformation depending on the transmembrane electrochemical potential difference of protons (delta mu H+). This conformational change is observable by measuring the change in the reactivity of Lys109 of the epsilon subunit of chloroplast-coupling-factor 1. Illumination of thylakoids increased the epsilon-Lys109 reactivity by a factor of 3-4 within 1 s. In the presence of ADP plus Pi, illumination of thylakoids increased the epsilon-Lys109 reactivity by a factor of only 2. Addition of ATP in the post-illumination dark or in the light after prior illumination increased the epsilon-Lys109 reactivity depending on the concentration of coexisting NH4Cl. ATP hydrolysis at high level was observed irrespective of the epsilon-Lys109 reactivity.

Involvement of arginine residues in the allosteric activation and inhibition of Synechocystis PCC 6803 ADPglucose pyrophosphorylase

ADPglucose pyrophosphorylase (EC 2.7.7.27) from the cyanobacterium Synechocystis PCC 6803 was desensitized to the effects of allosteric ligands by treatment with the arginine reagent, phenylglyoxal. Enzyme modification by phenylglyoxal resulted in inactivation when the enzyme was assayed under 3P-glycerate-activated conditions. There was little loss of the catalytic activity assayed in the absence of activator. Pi, 3P-glycerate, and pyridoxal-P were able to protect the enzyme from inactivation, whereas substrates gave minimal protection. The protective effect exhibited by Pi and 3P-glycerate was dependent on effector concentration. MgCl2 enhanced the protection afforded by 3P-glycerate. The enzyme partially modified by phenylglyoxal was more resistant to 3P-glycerate activation and Pi inhibition than the unmodified form. Vmax at saturating 3P-glycerate concentrations and the apparent affinity of the enzyme toward Pi were decreased upon phenylglyoxal modification. Incorporation of labeled phenylglyoxal into the enzyme was proportional to the loss of activity. Pi and 3P-glycerate nearly completely prevented incorporation of the reagent to the protein. Results suggest that one arginine residue per mol of enzyme subunit is involved in the binding of allosteric effector in the cyanobacterial ADPglucose pyrophosphorylase.

Chemical modification of rabbit skeletal muscle phosphorylase kinase with phenylglyoxal

Nonactivated phosphorylase kinase from rabbit skeletal muscle is inactivated by treatment with phenylglyoxal. Under mild reaction conditions, a derivative that retains 10-15% of the pH 8.2 catalytic activity is obtained. The kinetics of inactivation profile, differential effects of modification on pH 6.8 and 8.2 catalytic activities, and the insensitiveness of the modified enzyme to activation by ADP reveal that the 10-15% of catalytic activity remaining is very likely due to intrinsic catalytic activity of the derivative rather than to the presence of unmodified enzyme molecules. The kinetic results also suggest that the inactivation is correlatable with the reaction of one molecule of the reagent with the enzyme without any prior binding of phenylglyoxal. The phenylglyoxal modification reduces the autophosphorylation rate of the kinase. Autophosphorylated phosphorylase kinase is inactivated by phenylglyoxal at a much slower rate than the inactivation of nonactivated kinase. Thus, phenylglyoxal modification influences the phosphorylation and vice versa. The modified enzyme can be reactivated by treatment with trypsin or by dissociation using chatropic salts. The activity of the phenylglyoxal-modified enzyme after trypsin digestion or dissociation with LiBr reaches the same level as that of the native enzyme digested with trypsin or treated with LiBr under identical conditions. The results suggest that the effect of modification is overcome by dissociation of the subunits of phosphorylase kinase and that the catalytic site is not modified under conditions when 85% of the pH 8.2 catalytic activity is lost. Among various nucleotides and metal ions tested, only ADP, with or without Mg2+, afforded effective protection against inactivation with phenylglyoxal. At pH 6.8, 1 mM ADP afforded complete protection against inactivation. Experiments with 14C-labeled phenylglyoxal revealed that ADP seemingly protects one residue from modification. This result is in agreement with the kinetic result that the inactivation seemingly is due to reaction of one molecule of the reagent with the enzyme. The results confirm the existence of a high-affinity ADP binding site on nonactivated phosphorylase kinase and suggest the involvement of a functional arginyl residue at or near the ADP binding site in the regulation of of pH 8.2 catalytic activity of the enzyme.

Chemical modification of arginine residues of porcine muscle acylphosphatase

Acylphosphatase (acylphosphate phosphohydrolase, EC 3.6.1.7) from porcine skeletal muscle is inactivated by phenylglyoxal following pseudo-first-order kinetics. The dependence of the apparent first-order rate constant for inactivation on the phenylglyoxal concentration shows that the inactivation is also first order with respect to the reagent concentration. Among the competitive inhibitors for the enzyme examined, inorganic phosphate and ATP almost completely, and Cl- partially, protect the enzyme against the inactivation. The dissociation constants for inorganic phosphate and ATP determined from protection experiments by these inhibitors agree well with those from inhibition experiments by them. These results support the idea that the modification occurs at the phosphate-binding site. The amino-acid analysis reveals the lack of reaction at residues other than arginine. Circular dichroism spectra of the modified enzymes show that the inactivation seems not to be due to denaturation of the enzyme resulting from the modification of the non-essential arginine residues. The relationship between the loss of the enzyme activity and the number of arginine residues modified in the presence and absence of ATP shows that one arginine residue is possibly responsible for the inactivation of acylphosphatase.

Integrated functioning of the chloroplast coupling factor

This review is focused on some functional characteristics of the chloroplast coupling factor. The structure of the enzyme and the putative role of its subunits are recalled. An attempt is made to discriminate the driving force and the activator effects of the electrochemical proton gradient. Respective roles of delta pH, delta phi, external and internal pH are discussed with regard to mechanistic implications. The hypothesis of a functional switch of the enzyme between two states with better efficiency either in ATP synthesis or in ATP hydrolysis is also examined. A brief survey is made on some problems complicating quantitative studies of energy coupling, such as localized chemiosmosis, delta pH and delta phi computations, and scalar ATPases. The main data on the enzyme activation and the energy-dependent release of tightly bound nucleotides are summarized. The arguments for and against the catalytic competence of theses nucleotides are reviewed. Lastly, some prevailing models of the catalytic mechanism are presented. The relevance of nucleotides binding change events in this process is discussed.