Noncatalytic nucleotide binding sites: properties and mechanism of involvement in ATP synthase activity regulation

ATP synthases (FoF1-ATPases) of chloroplasts, mitochondria, and bacteria catalyze ATP synthesis or hydrolysis coupled with the transmembrane transfer of protons or sodium ions. Their activity is regulated through their reversible inactivation resulting from a decreased transmembrane potential difference. The inactivation is believed to conserve ATP previously synthesized under conditions of sufficient energy supply against unproductive hydrolysis. This review is focused on the mechanism of nucleotide-dependent regulation of the ATP synthase activity where the so-called noncatalytic nucleotide binding sites are involved. Properties of these sites varying upon free enzyme transition to its membrane-bound form, their dependence on membrane energization, and putative mechanisms of noncatalytic site-mediated regulation of the ATP synthase activity are discussed.

Individual biochemical behaviour versus biological robustness: spotlight on the regulation of cytochrome c oxidase

During evolution from prokaryotes to eukaryotes, the main function of cytochrome c oxidase (COX), i.e., the coupling of oxygen reduction to proton translocation without the production of ROS (reactive oxygen species) remained unchanged demonstrating its robustness. A new regulation of respiration by the ATP/ADP ratio was introduced in eukaryotes based on nucleotide interaction with the added COX subunit IV. This allosteric ATP-inhibition was proposed to keep the mitochondrial membrane potential (ΔΨ(m)) at low healthy values and thus prevents the formation of ROS at complexes I and III. ROS have been implicated in various degenerative diseases. The allosteric ATP-inhibition of COX is reversibly switched on and off by phosphorylation of COX at a serine or threonine. In more than 100 individual preparations of rat heart and liver mitochondria, prepared under identical conditions, the extent of allosteric ATP-inhibition varied. This variability correlates with the variable inhibition of uncoupled respiration in intact isolated mitochondria by ATP. It is concluded that in higher organisms the allosteric ATP-inhibition is continually switched on and off by neuronal signalling in order to change oxidative phosphorylation from optimal efficiency with lower rate of ATP synthesis under resting conditions (low ΔΨ(m) and ROS production) to maximal rate of ATP synthesis under active (working, stress) conditions (elevated ΔΨ(m) and ROS production).

Nucleotide binding to noncatalytic sites is essential for ATP-dependent stimulation and ADP-dependent inactivation of the chloroplast ATP synthase

Light-dependent binding of labeled ADP and ATP to noncatalytic sites of chloroplast ATP synthase and the effect of light-exposed thylakoid membrane preincubation with ADP or ATP on ATPase activity were studied. ADP binding during the preincubation was shown to inactivate the chloroplast ATPase, whereas ATP binding caused its activation. The rate and equilibrium constants of ATPase inactivation and activation were close to those of ADP and ATP binding to a noncatalytic site, with K (d) values of 38 and 33 μM, respectively. It is suggested that ADP- or ATP-binding to one of the noncatalytic sites affects the ATPase activity of chloroplast ATP synthase through a mechanism that modulates tightness of ADP binding to a catalytic site.