Photophosphorylation of base-modified nucleotide analogs by spinach chloroplasts

Proton slip of the chloroplast ATPase: its nucleotide dependence, energetic threshold, and relation to an alternating site mechanism of catalysis

The F-ATPase of chloroplasts couples proton flow to ATP synthesis, but is leaky to protons in the absence of nucleotides. This "proton slip" can be blocked by small concentrations of ADP or by inhibitors of the channel portion, CF0. We studied charge flow through the ATPase by flash spectrophotometry and analyzed the inhibition of proton slip by nucleotides, phosphate/arsenate, and insufficient proton motive force. The following inhibition constants (at given background concentrations) were observed: ADP, 0.2 microM (0.5 mM P(i)); ADP, 13.4 microM (no P(i)); P(i), 43 microM (1 microM ADP); GDP, 2.5 microM (0.5 mM P(i)); ATP, 2 microM. ADP and P(i) mutually lowered their respective inhibition constants. Phosphate could be replaced by arsenate. Proton slip occurred only if the proton motive force exceeded a certain threshold, similar to that for ATP synthesis. The inhibition of proton slip by ADP and GDP qualified the respective nucleotide binding sites as belonging to the subset of two (or three) potentially catalytic sites out of the total of six. We interpreted the ADP-induced transition between different conduction states of the ATPase from "slipping" to "closed" to "coupled" as a consequence of the alternating site mechanism of catalysis. Whereas the proton translocator idles in the absence of nucleotides, the high-affinity binding of the first ADP/P(i) couple to one site clutches proton flow to some (conformational) change that can only be executed after the binding of another ADP/P(i) couple to a second site. From there on these sites alternate in the catalytic cycle. An entropic machine is presented which likewise models proton slip, unisite, and multisite ATP synthesis and hydrolysis.

Inactivation of chloroplast H(+)-ATPase by modification of Lys beta 359, Lys alpha 176 and Lys alpha 266

Treatment of isolated, latent chloroplast ATPase with pyridoxal-5-phosphate (pyridoxal-P) in presence of Mg2+ causes inhibition of dithiothreitol-activated plus heat-activated ATP hydrolysis. The amount of [3H]pyridoxal-P bound to chloroplast coupling factor 1 (CF1) was estimated to run up to 6 +/- 1 pyridoxal-P/enzyme, almost equally distributed between the alpha- and beta-subunits. Inactivation, however, is complete after binding of 1.5-2 pyridoxal-P/CF1, suggesting that two covalently modified lysines prevent the activation of the enzyme. ADP as well as ATP in presence of Mg2+ protects the enzyme against inactivation and concomittantly prevents incorporation of a part of the 3H-labeled pyridoxal-P into beta- and alpha-subunits. Phosphate prevents labeling of the alpha-subunit, but has only a minor effect on protection against inactivation. The data indicate a binding site at the interface between the alpha- and beta-subunits. Cleavage of the pyridoxal-P-labeled subunits with cyanogen bromide followed by sequence analysis of the labeled peptides led to the detection of Lys beta 359, Lys alpha 176 and Lys alpha 266, which are closely related to proposed nucleotide-binding regions of the alpha- and beta-subunits.

Binding of 2'(3')-O-(2,4,6-trinitrophenyl)-adenosine 5'-diphosphate opens the pathway for protons through the chloroplast ATPase complex

The effect of 2'(3')-O-(2,4,6-trinitrophenyl)-adenosine 5'-diphosphate (TNP-ADP) on photophosphorylation and on the proton conductivity of the thylakoid membrane has been investigated. The results show that TNP-ADP is a potent competitive inhibitor of photophosphorylation (Ki = 1-2 microM). Moreover, in the absence of ADP and Pi, TNP-ADP accelerates basal electron transport of chloroplasts. Addition of ADP, which promotes release of the analogue from CF1, completely reverses this effect of TNP-ADP; likewise Pi alone reverses stimulation of electron transport by TNP-ADP. Dicyclohexylcarbodiimide treatment, which is known to close CF0 to H+, completely abolishes the effect of TNP-ADP. The measurements of the alkalization of the medium and the acidification of the thylakoid lumen following single turnover flashes showed that binding of TNP-ADP to CF1 increased membrane permeability for H+. Further results suggest that binding of TNP-ADP to the catalytic site of CF1 opens the CF0-CF1 complex for H+. Since ADP, as well as Pi alone, reverses the effect, it is concluded that TNP-ADP induces a conformation of the CF0-CF1 complex similar to the one triggered by simultaneous binding of ADP plus Pi. This may be achieved by interaction of the TNP residue with the Pi binding site. Thus it seems that the status of the catalytic site(s) in CF1 can be transmitted to the CF0 part to control proton flux through the ATPase complex in an economically reasonable way.

Localization of the tight ADP-binding site on the membrane-bound chloroplast coupling factor one

The photoaffinity analog 2-azido-ADP (2-azidoadenosine 5'-diphosphate) was used as a probe of the spinach chloroplast ATP synthase. The analog acted as a substrate for photophosphorylation. Several observations suggested that 2-azido-ADP and ADP bound to the same class of tight nucleotide binding sites: (a) 2-azido-ADP competitively inhibited ADP tight binding (Ki = 1.4 microM); (b) the concentration giving 50% maximum binding, K0.5 for analog tight binding (1 microM) was similar to that observed for ADP (2 microM); (c) nucleotide tight binding required prior membrane energization and was completely reversed by re-energization; (d) the tight binding of 2-azido-[beta-32P]ADP was completely prevented by ADP; (e) the analog inhibited the light-triggered ATPase activity at micromolar concentrations. Ultraviolet irradiation of washed thylakoid membranes containing tightly bound 2-azido-[beta-32P]ADP resulted in the covalent incorporation of the label into the membranes. Denaturing polyacrylamide gel electrophoresis of the labeled membranes demonstrated that the beta subunit of the coupling factor one complex was the only polypeptide in the thylakoid membranes which was labeled. These results identify the beta subunit of the coupling factor as the location of the tightly bound ADP on the thylakoid membranes.

Evaluation by steady-state enzyme kinetics of the role of tightly bound nucleotides during photophosphorylation

The ATP synthetase of chloroplast membranes binds ADP and ATP with high affinity, and the binding becomes quasi-irreversible under certain conditions. One explanation of the function of these nucleotides is that they are transiently tightly bound during ATP synthesis as part of the catalytic process, and that the release of tightly bound ATP from one catalytic site is promoted when ADP and P(i) bind to a second catalytic site on the enzyme. Alternatively, it is possible that the tightly bound nucleotides are not catalytic, but instead have some regulatory function. We developed steady-state rate equations for both these models for photophosphorylation and tested them with experiments where two alternative substrates, ADP and GDP, were phosphorylated simultaneously. It was impossible to fit the results to the equations that assumed a catalytic role for tightly bound nucleotides, whether we assumed that both ADP and GDP, or only ADP, are phosphorylated by a mechanism involving substrate-induced release of product from another catalytic site. On the other hand, the equations derived from the regulatory-site model that we tested were able to fit all the results relatively well and in an internally consistent manner. We therefore conclude that the tightly bound nucleotides most likely do not derive from catalytic intermediates of ATP synthesis, but that substrate (and possibly also product) probably bind both to catalytic sites and to noncatalytic sites. The latter may modulate the transition of the ATP-synthesizing enzyme complex between its active and inactive states.

Adenosine di- and triphosphate transport in mitochondria. Role of the amidine region for substrate binding and transport

A variety of base-modified nucleotide analogues was prepared and characterized as their alpha-32P- or U-14C-labeled compounds. Carrier-linked nucleotide binding and carrier-catalyzed exchange across the inner membrane of rat liver mitochondria were measured by using an inhibitor (atractyloside) stop method. Kinetic data of carrier-specific bound analogues were evaluated from Dixon plots and indicate that these analogues are competitive inhibitors for mitochondrial [14C]ADP uptake. Km and Vmax values for carrier-mediated uptake of nucleotide analogues were calculated from Lineweaver-Burk plots. By means of the analogues, a systematic mapping of the essential chemical and steric interactions between the transporter protein and the heterocycle of its substrate in the course of the binding as well as transfer step was achieved. Prerequisites for carrier-specific binding (recognition) are (A) an anti- or syn-positioned beta-glycosyl-linked heterocycle, (B) a nitrogen ring atom in position 7 for syn-structured analogues, and (C) an electron-rich region at the N(1) position, i.e., a permanent dipole moment oriented toward N(1) for anti-structured analogues. Additional requirements for subsequent transport catalysis are (A) a non-fixed anti-positioned base moiety with a beta-glycosyl torsion angle of about -20 degrees, (B) a C(6)-positioned amino group, and (C) an unsubstituted C(2) atom. The complementary binding site at the carrier protein to the N(1)-C(6)(-NH2) amidine region is proposed to be represented by two juxtaposed and invariant bonding points, i.e., an asparagine or glutamine residue.

Nucleotidic cofactors and the origin of the genetic code

In this article it is suggested that the first coding system started from specific interactions between the nucleotide part of nucleotidic cofactors and enzymes. These interactions generated a first primitive code of four words of one letter each for the four primittive amino acids (phenylalanine, lysine, glycine and proline); when the triplet code (which allowed the integration of 20 amino acids into proteins) progressively appeared, it must have been modulated by the existence of this first coding system.