In Saccharomyces cerevisiae, cytosolic NADH generated during biomass production must be efficiently reoxidized to sustain metabolic flux. Under aerobic conditions, this process is primarily mediated by the oxidative phosphorylation and specifically via external NADH dehydrogenases (Nde1p/Nde2p) and the glycerol-3-phosphate (G3P) shuttle. Although these pathways perform equivalent redox functions, their relative contributions vary with physiological conditions, indicating the existence of regulatory mechanisms governing electron partitioning.Here, we combined experimental measurements and kinetic modeling to investigate the mechanisms underlying electron competition between NADH and G3P oxidation in isolated yeast mitochondria. Using wild-type and Δnde1 strains, we determined the kinetic parameters of the relevant dehydrogenases and analyzed their dependence on the quinone redox state. A mathematical model incorporating these parameters reproduced the experimentally observed preferential utilization of NADH in wild-type mitochondria as well as the simultaneous oxidation of NADH and G3P in the Δnde1 mutant.Our results demonstrate that, under conditions where the downstream respiratory chain is rate-limiting, electron partitioning is governed by the ratio of catalytic efficiencies (Vmax/K0.5) of NADH and G3P dehydrogenases with respect to the quinone redox ratio (Q/QH₂).These findings identify a simple kinetic principle underlying substrate competition in mitochondrial electron transport and provide a quantitative framework for understanding metabolic flux distribution in eukaryotic cells.
In eukaryotic light-harvesting complexes, the protein host tunes chlorophyll-a (Chl a) site energies through a number of mechanisms (e.g. hydrogen bonding; excitonic/charge-transfer (CT) mixing). However, whether protein-imposed macrocycle distortions also contribute to fine-tuning this energy remains unclear. We have investigated three Chl a-only algal antennas - VCP (from Nannochloropsis oceanica), rVCP (Trachydiscus minutus), and CLH (Chromera velia). These complexes show distinct red-shifted absorption features, which at 77 K range from 670 nm up to an extremely red-shifted band at 700 nm (rVCP). By analysis of marker bands in excitation-selective resonance Raman spectra, we have determined the contribution of (i) hydrogen bonding to the keto group (1630–1700 cm−1 region), and/or (ii) macrocycle distortion (bands near 1555 and 1612 cm−1) to these red-shifts. Finally, the absence of vibrational features in fluorescence line-narrowing measurements for the reddest rVCP band is consistent with strong excitonic coupling, which we suggest is at the origin of this large red shift. Our results indicate that tuning of the functional properties of Chl a antennas may occur via distortion of the macrocycle.
center dot The VDE-dependent component of NPQ induction increases linearly with temperature between 10 and 35 degrees C. center dot The ZEP3-dependent component of NPQ relaxation follows a Gaussian-like temperature response, peaking at 25.8 f 0.1 degrees C. center dot NPQ relaxation is fastest at 25.8 degrees C, with the ZEP3-dependent rate constant declining at both lower and higher temperatures.
How did phosphate become the universal energetic currency of life? Traditional approaches to phosphorylation in early evolution studies entail non-physiological conditions. Phosphite, HPO32-, which is oxidized enzymatically by many microbes and which naturally occurs in serpentinizing hydrothermal vents, readily phosphorylates ribose, glucose, glycerol, serine, AMP, creatine and acetate to generate phosphoester, phosphoanhydride and acylphosphate bonds in hours to days at 25-100 degrees C in pure alkaline water. These reactions are thermodynamically favorable because anoxic phosphite oxidation to phosphate and H-2 is highly exergonic. But the reactions do not proceed without catalysts. The most effective catalyst yet identified is a nanoparticular form of a shiny metal: zero-valent (native, or elemental) palladium (Pd-0). Native palladium, like phosphite, also naturally occurs in serpentinizing hydrothermal vents, as do other native platinum group elements (PGE), including Pt, Rh, Ru and Ir. Here we survey the use of standard currencies of biochemical energy used in core biosynthesis of amino acids, bases and cofactors from H-2, CO2, NH3, H2S, H2O and P-i, their frequency is phosphate > redox > thioesters. We test PGE as catalysts of phosphite oxidation and phosphorylation. Although all metals tested readily oxidize phosphite, only Pd-0 efficiently catalyzes phosphorylation, generating phosphorylated products at physiologically relevant concentrations. In phosphorylation reactions via phosphite oxidation, a portion of the energy released is conserved in phosphorylated products, as in biological energy conservation. Favorable thermodynamics of aqueous phosphite-dependent phosphorylation reactions suggest their possible involvement in the origin of metabolism.
Cyanobacterial NDH-1 complexes use a conserved Type-I energy-transducing core for respiratory electron transfer, photosystem I cyclic electron flow, and, in specialized isoforms, active CO2 uptake. This review examines how the common structural and energetic principles of respiratory Complex I may be adapted to these distinct physiological functions. Building on current mechanistic models, this review frames quinone reduction as producing a distributed, metastable "tense state" in the membrane arm. This state is represented not by a single strained conformation, but by coupled changes in proton affinity, ion-pair geometry, hydration, and local electrostatics. This framework is then applied to the cyanobacterial CO2-uptake complexes NDH-13 and NDH-14, in which the distal NdhF3/NdhF4-CupA/B module appears to repurpose part of the canonical proton-coupling machinery for Zn-dependent CO2 hydration. Bioenergetic estimates indicate that ferredoxin-dependent plastoquinone reduction could, in principle, support three-proton translocation together with CO2 hydration and bicarbonate accumulation. The model connects Complex I energy transduction pathways to cyanobacterial CO2 hydration chemistry and identifies testable predictions regarding coupling at the Zn-site to distant plastoquinone reduction and proton transfer.
