BBA - Bioenergetics (v.1757, #3)

Reply to Pettai et al.'s response by Hans-W. Trissl (160).

Modeling of excitation transfer pathways have been carried out for the structure of Spirulina platensis C-phycocyanin. Calculations by Förster mechanism using the crystal structure coordinates determined in our laboratory indicate ultra-fast lateral energy transfer rates between pairs of chromophores attached to two adjacent hexamer disks. The pairwise transfer times of the order of a few pico-seconds correspond to resonance transitions between peripheral β155 chromophores. A quantitative lateral energy transfer model for C-phycocyanin light-harvesting antenna rods that is suggestive to its native structural organization emerges from this study.
Keywords: Light-harvesting; Photosynthesis; Phycobilisomes; Phycocyanobilin;

Mitochondrial metabolic states and membrane potential modulate mtNOS activity by Laura B. Valdez; Tamara Zaobornyj; Alberto Boveris (166-172).
The mitochondrial metabolic state regulates the rate of NO release from coupled mitochondria: NO release by heart, liver and kidney mitochondria was about 40–45% lower in state 3 (1.2, 0.7 and 0.4 nmol/min mg protein) than in state 4 (2.2, 1.3 and 0.7 nmol/min mg protein). The activity of mtNOS, responsible for NO release, appears driven by the membrane potential component and not by intramitochondrial pH of the proton motive force. The intramitochondrial concentrations of the NOS substrates, l-arginine (about 310 μM) and NADPH (1.04–1.78 mM) are 60–1000 times higher than their K M values. Moreover, the changes in their concentrations in the state 4–state 3 transition are not enough to explain the changes in NO release. Nitric oxide release was exponentially dependent on membrane potential as reported for mitochondrial H2O2 production [S.S. Korshunov, V.P. Skulachev, A.A. Satarkov, High protonic potential actuates a mechanism of production of reactive oxygen species in mitochondria. FEBS Lett. 416 (1997) 15–18.]. Agents that decrease or abolish membrane potential minimize NO release while the addition of oligomycin that produces mitochondrial hyperpolarization generates the maximal NO release. The regulation of mtNOS activity, an apparently voltage-dependent enzyme, by membrane potential is marked at the physiological range of membrane potentials.
Keywords: Mitochondrial NO; NO release; mtNOS; State 4–state 3 transition; Voltage-dependent enzyme activity; Mitochondrial membrane potential;

The increase of chlorophyll fluorescence yield in chloroplasts in a 12.5 Hz train of saturating single turnover flashes and the kinetics of fluorescence yield decay after the last flash have been analyzed. The approximate twofold increase in F m relative to F o, reached after 30–40 flashes, is associated with a proportional change in the slow (1–20 s) component of the multiphasic decay. This component reflects the accumulation of a sizeable fraction of QB-nonreducing centers. It is hypothesized that the generation of these centers occurs in association with proton transport across the thylakoid membrane. The data are quantitatively consistent with a model in which the fluorescence quenching of QB-nonreducing centers is reversibly released after second excitation and electron trapping on the acceptor side of Photosystem II.
Keywords: Chlorophyll a; Fluorescence quenching; Photosystem 2; Three state trapping mechanism (TSTM); Non-QB-reducing center; Single turnover excitation;

Photodissociation of the fully reduced carbonmonoxy bound cytochrome aa 3 from Rb. sphaeroides results in ultrafast ligand transfer between heme a 3 and CuB, which is followed by thermal dissociation from CuB on longer time scales. We have utilized photoacoustic calorimetry to obtain a detailed thermodynamic description of the mechanism of ligand photodissociation and transfer between heme a3 and CuB. Subsequent to ligand photodissociation an additional process, which has not been characterized previously, was observed with the lifetime of 485 ns at 18 °C and is coupled to a volume expansion of 3.3 ml mol−1. From the temperature dependence, an activation barrier of 4 kcal mol−1 was determined. We attribute the observed 500 ns process to changes in CuB ligation subsequent to ligand translocation. In a photoacoustic study on CO photodissociation from bovine heart aa 3 oxidase, no volume changes were observed on the ns timescale, indicating that a different mechanism may control ligand dissociation and binding within the binuclear center of the bacterial and bovine enzymes.
Keywords: Photoacoustic calorimetry; Cytochrome c oxidase; CO photolysis; Respiratory protein;

The role of carboxylic residues at the high-affinity, Mn-binding site in the ligation of iron cations blocking the site [Biochemistry 41 (2000) 5854] was studied, using a method developed to extract the iron cations blocking the site. We found that specifically bound Fe(III) cations can be extracted with citrate buffer at pH 3.0. Furthermore, citrate can also prevent the photooxidation of Fe(II) cations by YZ . Participation of a COOH group(s) in the ligation of Fe(III) at the high-affinity site was investigated using 1-ethyl-3-[(3-dimethylamino)propyl] carbodiimide (EDC), a chemical modifier of carboxylic amino acid residues. Modification of the COOH groups inhibits the light-induced oxidation of exogenous Mn(II) cations by Mn-depleted photosystem II (PSII[−Mn]) membranes. The rate of Mn(II) oxidation saturates at ≥10 μM in PSII(−Mn) membranes and ≥500 μM in EDC-treated PSII (−Mn) samples. Intact PSII(−Mn) membranes have only one site for Mn(II) oxidation via YZ (dissociation constant, K d  = 0.64 μM), while EDC-treated PSII(−Mn) samples have two sites (K d  = 1.52 and 22 μM; the latter is the low-affinity site). When PSII(−Mn) membranes were incubated with Fe(II) before modifier treatment (to block the high-affinity site) and the blocking iron cations were extracted with citrate (pH 3.0) after modification, the membranes contained only one site (K d  = 2.3 μM) for exogenous Mn(II) oxidation by YZ radical. In this case, the rate of electron donation via YZ saturated at a Mn(II) concentration ≥15 μM. These results indicate that the carboxylic residue participating in Mn(II) coordination and the binding of oxidized manganese cations at the HAZ site is protected from the action of the modifier by the iron cations blocking the HAZ site. We concluded that the carboxylic residue (D1 Asp-170) participating in the coordination of the manganese cation at the HAZ site (Mn4 in the tetranuclear manganese cluster [Science 303 (2004) 1831]) is also involved in the ligation of the Fe cation(s) blocking the high-affinity Mn-binding site.
Keywords: Photosystem II; PSII; High-affinity site; Manganese; Iron; D1 Asp-170; Citrate; Chemical modification; Carboxylic amino acid; Low pH; Metal ion oxidation; Yz; Metal ion binding; Low affinity site;

We demonstrated recently that, in intact cells of Chlamydomonas reinhardtii, interruption of CO2 fixation via the Calvin cycle inhibits the synthesis of proteins in photosystem II (PSII), in particular, synthesis of the D1 protein, during the repair of PSII after photodamage. In the present study, we investigated the mechanism responsible for this phenomenon using intact chloroplasts isolated from spinach leaves. When CO2 fixation was inhibited by exogenous glycolaldehyde, which inhibits the phosphoribulokinase that synthesizes ribulose-1,5-bisphosphate, the synthesis de novo of the D1 protein was inhibited. However, when glycerate-3-phosphate (3-PGA), which is a product of CO2 fixation in the Calvin cycle, was supplied exogenously, the inhibitory effect of glycolaldehyde was abolished. A reduced supply of CO2 also suppressed the synthesis of the D1 protein, and this inhibitory effect was also abolished by exogenous 3-PGA. These findings suggest that the supply of 3-PGA, generated by CO2 fixation, is important for the synthesis of the D1 Protein. It is likely that 3-PGA accepts electrons from NADPH and decreases the level of reactive oxygen species, which inhibit the synthesis of proteins, such as the D1 protein.
Keywords: Calvin cycle; CO2 fixation; D1 protein; Glycerate-3-phosphate; Photoinhibition; Photosystem II;

A functionally inactive, cold-stabilized form of the Escherichia coli F1Fo ATP synthase by Mikhail A. Galkin; Robert R. Ishmukhametov; Steven B. Vik (206-214).
An unusual effect of temperature on the ATPase activity of E. coli F1Fo ATP synthase has been investigated. The rate of ATP hydrolysis by the isolated enzyme, previously kept on ice, showed a lag phase when measured at 15 °C, but not at 37 °C. A pre-incubation of the enzyme at room temperature for 5 min completely eliminated the lag phase, and resulted in a higher steady-state rate. Similar results were obtained using the isolated enzyme after incorporation into liposomes. The initial rates of ATP-dependent proton translocation, as measured by 9-amino-6-chloro-2-methoxyacridine (ACMA) fluorescence quenching, at 15 °C also varied according to the pre-incubation temperature. The relationship between this temperature-dependent pattern of enzyme activity, termed thermohysteresis, and pre-incubation with other agents was examined. Pre-incubation of membrane vesicles with azide and Mg2+, without exogenous ADP, resulted in almost complete inhibition of the initial rate of ATPase when assayed at 10 °C, but had little effect at 37 °C. Rates of ATP synthesis following this pre-incubation were not affected at any temperature. Azide inhibition of ATP hydrolysis by the isolated enzyme was reduced when an ATP-regenerating system was used. A gradual reactivation of azide-blocked enzyme was slowed down by the presence of phosphate in the reaction medium. The well-known Mg2+ inhibition of ATP hydrolysis was shown to be greatly enhanced at 15 °C relative to at 37 °C. The results suggest that thermohysteresis is a consequence of an inactive form of the enzyme that is stabilized by the binding of inhibitory Mg-ADP.
Keywords: F1Fo; ATP synthase; Thermohysteresis; Azide; Mg-ADP inhibition;

All transhydrogenases appear to have three components: dI, which binds NAD(H), and dIII, which binds NADP(H), protrude from the membrane, and dII spans the membrane. However, the polypeptide composition of the enzymes varies amongst species. The transhydrogenases of Mycobacterium tuberculosis and of Rhodospirillum rubrum have three polypeptides. Sequence analysis indicates that an ancestral three-polypeptide enzyme evolved into transhydrogenases with either two polypeptides (such as the Escherichia coli enzyme) or one polypeptide (such as the mitochondrial enzyme). The fusion steps in each case probably led to the development of an additional transmembrane helix. A hybrid transhydrogenase was constructed from the dI component of the M. tuberculosis enzyme and the dII and dIII components of the R. rubrum enzyme. The hybrid catalyses cyclic transhydrogenation but not the proton-translocating, reverse reaction. This shows that nucleotide-binding/release at the NAD(H) site, and hydride transfer, are fully functional but that events associated with NADP(H) binding/release are compromised. It is concluded that sequence mismatch in the hybrid prevents a conformational change between dI and dIII which is essential for the step accompanying proton translocation.
Keywords: Transhydrogenase; Rhodospirillum rubrum; Mycobacterium tuberculosis; Proton translocation; Nicotinamide nucleotides; Phylogenetic tree;