Lectures 22/23: Redoxreactions and Oxidative Phosphorylation Flashcards
Glutamate dehydrogenase
Catalyzes the reversible conversion of ketoglutarate and glutamate
Can be cataplerotic or anaplerotic
Pyruvate carboxylase
Catalyses irreversible reaction of pyruvate to oxaloacetate
Anaplerotic and gluconeogenic enzyme
Anaplerotic carboxylation
Conversion of pyruvate to oxaloacetate by pyruvate carboxylase
Acetyl-CoA
Oxidation of pyruvate to acetyl-CoA is irreversible
High levels inhibit pyruvate dehydrogenase
High levels activate pyruvate carboxylase: converted to citric acid cycle intermediates that are glucogenic
Glucogenic
Metabolites that can be converted to glucose through gluconeogenesis
Ketogenic
Metabolites that cannot be converted to glucose through gluconeogenesis
Oxidation
Loss of electrons
Oxidation NADH and QH2 generate ATP
Reduction
Gain of electrons
Redox through transfer of a hydride ion
Niacin
Vitamin B3
Nicotinamide
Nicotinamide adenine dinucleotide
NAD+
NADH carries two electrons that it can give up easily
In oxidative phosphorylation, reduces O2 to H2O to drive formation of ATP
FAD
Accepts two protons and two electrons to become FADH2
No change in charge of the molecule
Riboflavin (vitamin B12)
FADH2 reduced Q to QH2: carries two electrons that it can give up easily
In oxidative phosphorylation, reduces O2 to H2O to drive formation of ATP
Reduction potential
Tendency of a substance to accept electrons to become reduced
Measured in volts
Higher means that substance is more easily reduces and is a stronger oxidant
Rejects energy change that would occur if electrons were transferred
Written as a half reaction
Standard reduction potential
Reduction of potential of substances under standard conditions
Standard reduction potential E*’ is a characteristic of each redox active substance and reflects its affinity for electrons
Oxidation potential
Opposite in sign to standard reduction potential
Positive reduction potential
Higher: greater tendency to accept electrons and therefore become reduced
Negative reduction potential
Most negative: least tendency to accept electrons and become reduced
Electrons flow spontaneously from a species with a more negative E’ to a species with a more positive E’
Nernst Equation
Defines actual reduction potential
deltaE*’
deltaE’= E’ (e acceptor) - E*’ (e donor)
Spontaneous when positive
Standard free energy change
deltaG’= -nFdeltaE’
Spontaneous when deltaE’ is positive and deltaG’ is negative
Oxidative phosphorylation
Takes place in mitochondria
Occurs over the inner membrane
Proteins accumulate in inter membrane space
Series of redox reactions generates protein gradient to fuel ATP synthesis: electrons passed down electron transport chain of complexes I-IV
Protons flow back into mitochondrial matrix through complex V (ATP synthase) and fuel the synthesis of ATP
Inner membrane
Proton-Rich
Impermeable to several metabolites (ATP, ADP) and ions (H, OH, K, Cl, Phosphate) and fully permeable to O2, H20, CO2
Compartmentation of mitochondria
Allows pathway control through controlling the localization of metabolites
Special transport systems to transport metabolites
NADH made during glycolysis must get to the mitochondrial matrix to by reoxidizes and ATP made in mitochondrial matrix must be transported into the cytosol (ADP and P must get to matrix from cytosol)
Malate-aspartate shuttle
Interaction of cytosolic malate dehydrogenase and matrix malate dehydrogenase to transport NADH to mitochondrial matrix via oxidation of malate to oxaloacetate
Complex I
Transfers electrons from NADH to H and transports 4H into inter membrane space
Energy release by oxidation of NADH used to transport H using proton pump
H transport is against concentration and charge gradient: requires energy
NADH - FMN - Fe-S - Q
Coenzyme Q
Hydrophobic and remains inside lipid bilayer
FMN
Redox active cofactor
Transport electrons
Related to FAD
Complex II
Succinate dehydrogenase from citric acid cycle
Oxidation of succinate to fumarate and reduction of FAD to FADH2
Oxidation of FADH2 to FAD and reduction of Q to QH2
Fatty acid oxidation
Produces QH2
Glycerol-3-phosphate shuttle
Two redox reactions catalyzed by glycerol-3-phosphate dehydrogenase
- Reduction of 1,3-bisphosphoglycerate to glycerol-3P
- Deoxidation of DHAP to transfer electron Q
Complex III
Two electrons from QH2 reduce two molecules of cytochrome C
Reduced cytochrome c moves to Complex IV
Q returns to complex I and complex II
Four protons are pumped to intermembrane space
Cytochrome
Contains heme prosthetic group
Undergoes reversible one-electron transfers
Oxidized to Fe3+ or reduced to Fe2+
Membrane soluble
Transfers one electron at a time from Complex III to Complex IV
Complex IV
Cytochrome C oxidase
Oxidizes cytochrome C and reduces O2
For every 2 electrons donated by cytochrome C, two protons are translocated into the intermembrane space
Sometimes oxygen escapes after receiving only one electrons: free radicals
Free radicals
When oxygen takes up only 1 electron
Forms superoxide radical anion
Highly reactive, can damage nucleic acid, proteins and lipids
Cumulative damage from free radicals is thought to contribute to many diseases and aging
Proton gradient
Source of free energy through proton motive force
Potential free energy due to chemical and charge imbalance
ATP synthase taps into the electrochemical proton gradient to phosphorylate ADP
Complex V
ATP synthase (F1F0-ATPase)
Uses electrochemical gradient to provide free energy for phosphorylation
Needs ADP and P in mitochondrial matrix
ATP translocase
Imports ADP into matrix and exports ATP
Phosphate transporter
Brings P and H into matrix
F0
Transmembrane portion of ATP synthase
Blocked by antibiotic oligomycin
H binds to C subunit
C subunit moves away from A subunit, when a new C reaches A subunit H is released
One rotation of ring translocates 8 protons
F1
Water soluble peripheral portion that extends into matrix
Catalyzes the phosphorylation of ADP and ATP
3 alternating alpha and beta form a hexameter around the end of the gamma subunit
F1 beta subunit
Binds to ADP/ATP
Forms Open, Tight and Loose with alpha subunit as the gamma subunit rotates
At any given time, there is one alpha-beta in each conformation
P:O ratio
# of phosphorylations of ADP per # of oxygen atoms reduced Not a whole number: conversion of energy
Rate of oxidative phosphorylation
Depends on rate of fuel catabolism
Regulated by the availability of reduced cofactor produced by other metabolic processes
Efficiency of coupling between electron transport chain and ATP synthesis
If proton gradient is used to fuel other processes or if H are transported back over membrane, less ATP synthesized per oxygen
Uncoupling
Protons flowing into matrix without powering ATP synthase
Proton motive force is dissipated
NADH is oxidized, electrons are transported and oxygen is reduced to water but ATP is not made
Mediated by uncoupling proteins or chemicals
Some is always ongoing: protective by reducing oxygen species production
Generates heat
Uncoupling proteins
Mediates uncoupling
Dinitrophenol
Causes uncoupling
FCCP
Causes uncoupling
Non-shivering thermogenesis
Heat caused by uncoupling
In brown adipose tissue
