BBA - Bioenergetics (v.1604, #1)

Bcl-2 and tBid proteins counter-regulate mitochondrial potassium transport by Roman A. Eliseev; Jason D. Salter; Karlene K. Gunter; Thomas E. Gunter (1-5).
The mechanism of cytochrome c release from mitochondria in apoptosis remains obscure, although it is known to be regulated by bcl-2 family proteins. Here we describe a set of novel apoptotic phenomena—stimulation of the mitochondrial potassium uptake preceding cytochrome c release and regulation of such potassium uptake by bcl-2 family proteins. As a result of increased potassium uptake, mitochondria undergo moderate swelling sufficient to release cytochrome c. Overexpression of bcl-2 protein prevented the mitochondrial potassium uptake as well as cytochrome c release in apoptosis. Bcl-2 was found to upregulate the mitochondrial potassium efflux mechanism—the K/H exchanger. Specific activation of the mitochondrial K-uniporter led to cytochrome c release, which was inhibited by bcl-2. tBid had an opposite effect—it stimulated mitochondrial potassium uptake resulting in cytochrome c release. The described counter-regulation of mitochondrial potassium transport by bcl-2 and Bid suggests a novel view of a mechanism of cytochrome c release from mitochondria in apoptosis.
Keywords: Apoptosis; Mitochondrion; Cytochrome c; Potassium transport; bcl-2; tBid;

The temperature dependence of the biphasic electron transfer (ET) from the secondary acceptor A1 (phylloquinone) to iron–sulfur cluster FX was investigated by flash absorption spectroscopy in photosystem I (PS I) isolated from Synechocystis sp. PCC 6803. While the slower phase (τ=340 ns at 295 K) slowed upon cooling according to an activation energy of 110 meV, the time constant of the faster phase (τ=11 ns at 295 K) was virtually independent of temperature. Following a suggestion in the literature that the two phases arise from bidirectional ET involving two symmetrically arranged phylloquinones, QK-A and QK-B, it is concluded that energetic parameters (most likely the driving forces) rather than the electronic couplings are different for ET from QK-A to FX and from QK-B to FX. Two alternative schemes of ET in PS I are presented and discussed.
Keywords: Electron transfer; Photosystem I; Phylloquinone; Activation energy; Transient absorption spectroscopy;

The activation barrier (the activation free energy) for the reaction's elementary act proper does not depend on the presence of reactants outside the reaction complex. The barrier is determined directly by the concentration-independent configurational free energy. In the case of redox reactants with pH-dependent redox potential, only the pH-independent quantity, the configurational redox potential enters immediately into expression for activation energy. Some typical cases of such reactions have been discussed (e.g., simultaneous proton and electron detachment, acid dissociation followed by oxidation, dissociation after oxidation, and others). For these mechanisms, the algorithms for calculation of the configurational redox potential from the experimentally determined redox potentials have been described both for the data related to a dissolved reactant or to a prosthetic group of an enzyme. Some examples of pH-dependent enzymatic redox reactions, in particular for the Rieske iron–sulfur protein, have been discussed.
Keywords: Configurational free energy; Activation energy; Electron transfer; Proton transfer; Cytochrome bc 1 complex; Rieske protein;

Acceptor and donor-side interactions of phenolic inhibitors in Photosystem II by Arthur G. Roberts; Wolfgang Gregor; R.David Britt; David M. Kramer (23-32).
Certain phenolic compounds represent a distinct class of Photosystem (PS) II QB site inhibitors. In this paper, we report a detailed study of the effects of 2,4,6-trinitrophenol (TNP) and other phenolic inhibitors, bromoxynil and dinoseb, on PS II energetics. In intact PS II, phenolic inhibitors bound to only 90–95% of QB sites even at saturating concentrations. The remaining PS II reaction centers (5–10%) showed modified QA to QB electron transfer but were sensitive to urea/triazine inhibitors. The binding of phenolic inhibitors was 30- to 300-fold slower than the urea/triazine class of QB site inhibitors, DCMU and atrazine. In the sensitive centers, the S2QA state was 10-fold less stable in the presence of phenolic inhibitors than the urea/triazine herbicides. In addition, the binding affinity of phenolic herbicides was decreased 10-fold in the S2QA state than the S1QA state. However, removal of the oxygen-evolving complex (OEC) and associated extrinsic polypeptides by hydroxylamine (HA) washing abolished the slow binding kinetics as well as the destabilizing effects on the charge-separated state. The S2-multiline electron paramagnetic resonance (EPR) signal and the ‘split’ EPR signal, originating from the S2YZ state showed no significant changes upon binding of phenolic inhibitors at the QB site. We thus propose a working model where QA redox potential is lowered by short-range conformational changes induced by phenolic inhibitor binding at the QB niche. Long-range effects of HA-washing eliminate this interaction, possibly by allowing more flexibility in the QB site.
Keywords: Conformational change; Phenolic inhibitor; QA redox potential; Manganese cluster; Electron paramagnetic resonance; Redox-active tyrosine;

A reduced model of the fluorescence from the cyanobacterial photosynthetic apparatus designed for the in situ detection of cyanobacteria by M. Beutler; K.H. Wiltshire; M. Arp; J. Kruse; C. Reineke; C. Moldaenke; U.-P. Hansen (33-46).
Fluorometric determination of the chlorophyll (Chl) content of cyanobacteria is impeded by the unique structure of their photosynthetic apparatus, i.e., the phycobilisomes (PBSs) in the light-harvesting antennae. The problems are caused by the variations in the ratio of the pigment PC to Chl a resulting from adaptation to varying environmental conditions. In order to include cyanobacteria in fluorometric analysis of algae, a simplified energy distribution model describing energy pathways in the cyanobacterial photosynthetic apparatus was conceptualized. Two sets of mathematical equations were derived from this model and tested. Fluorescence of cyanobacteria was measured with a new fluorometer at seven excitation wavelength ranges and at three detection channels (650, 685 and 720 nm) in vivo. By employing a new fit procedure, we were able to correct for variations in the cyanobacterial fluorescence excitation spectra and to account for other phytoplankton signals. The effect of energy-state transitions on the PC fluorescence emission of PBSs was documented. The additional use of the PC fluorescence signal in combination with our recently developed mathematical approach for phytoplankton analysis based on Chl fluorescence spectroscopy allows a more detailed study of cyanobacteria and other phytoplankton in vivo and in situ.
Keywords: Phycobilisome; Energy distribution model; Phycocyanin; Cyanobacteria; Fluorescence; Phytoplankton;

The heme domain of cellobiose oxidoreductase: a one-electron reducing system by Maria G Mason; Peter Nicholls; Christina Divne; B.Martin Hallberg; Gunnar Henriksson; Michael T Wilson (47-54).
Phanerochaete chrysosporium cellobiose oxidoreductase (CBOR) comprises two redox domains, one containing flavin adenine dinucleotide (FAD) and the other protoheme. It reduces both two-electron acceptors, including molecular oxygen, and one-electron acceptors, including transition metal complexes and cytochrome c. If the latter reacts with the flavin, the reduced heme b acts merely as a redox buffer, but if with the b heme, enzyme action involves a true electron transfer chain. Intact CBOR fully reduced with cellobiose, CBOR partially reduced by ascorbate, and isolated ascorbate-reduced heme domain, all transfer electrons at similar rates to cytochrome c. Reduction of cationic one-electron acceptors via the heme group supports an electron transfer chain model. Analogous reactions with natural one-electron acceptors can promote Fenton chemistry, which may explain evolutionary retention of the heme domain and the enzyme's unique character among secreted sugar dehydrogenases.
Keywords: Cellobiose; Cellobiose dehydrogenase; Heme domain; Ascorbate; Cellobiose oxidoreductase; Cytochrome c; Heme b; Electron transfer; Lignin; Phanerochaete chrysosporium;