Substrate-level phosphorylation is a type of chemical reactionthat results in the formation and creation of adenosine triphosphate(ATP) by the direct transfer and donation of a phosphoryl(PO3) group to adenosine diphosphate(ADP) from a reactive intermediate. While technically the transfer is PO3, or a phosphoryl group, convention in biological sciences is to refer to this as the transfer of a phosphate group. In cells, it occurs primarily and firstly in the cytoplasm (in glycolysis) under both aerobic and anaerobicconditions.
Unlike oxidative phosphorylation, here the oxidation and phosphorylation are not coupled or joined, although both types of phosphorylation result in ATP. • It should be noted that there is an oxidation reaction coupled to phosphorylation, however this occurs in the generation of 1,3-bisphosphoglycerate from 3-phosphoglyceraldehyde via a dehydrogenase. ATP is generated in a separate step (key difference from oxidative phosphorylation) by transfer of the high energy phosphate on 1,3-bisphosphoglycerate to ADP via a kinase.
ATP is synthesized when protons flow back to the mitochondrial matrix through an enzyme complex ATP synthase. The oxidation of fuels and the phosphorylation of ADP are coupled by a proton gradient across the inner mitochondrial membrane. Oxidative phosphorylation is the process in which ATP is formed as a result of the transfer of electrons from NADH or FADH2 to O2 by a series of electron carriers.
OXIDATIVE PHOSPHORYLATION IN EUKARYOTES TAKES PLACE IN MITOCHONDRIA Two membranes: outer membrane inner membrane (folded into cristae) Two compartments: (1) the intermembrane space (2) the matrix Location of mitochondrial complexes • Inner mitochondrial membrane: Electron transport chainATP synthase • Mitochondrial matrix:Pyruvate dehydrogenase complexCitric acid cycleFatty acid oxidation The outer membrane is permeable to small molecules and ions because it contains pore-forming protein (porin). The inner membrane is impermeable to ions and polar molecules. Contains transporters (translocases).
NADH FMN Fe-S Co-Q Fe-S cyt c1 cyt c cyt a cyt a3 O2 cyt b succinate FAD Fe-S THE ELECTRON TRANSPORT CHAIN Series of enzyme complexes (electron carriers) embedded in the inner mitochondrial membrane, which oxidize NADH2 and FADH2 and transport electrons to oxygen is calledrespiratory electron-transport chain (ETC). The sequence of electron carriers in ETC
High-Energy Electrons: Redox Potentials and Free-Energy Changes In oxidative phosphorylation, the electron transfer potentialof NADH or FADH2 is converted into the phosphoryl transfer potentialof ATP. Phosphoryl transfer potential is G°' (energy released during the hydrolysis of activated phos-phate compound). G°' for ATP = -7.3 kcal mol-1 Electron transfer potential is expressed as E'o, the (also called redox potential, reduction potential, or oxidation-reduction potential).
NADH FMN Fe-S Co-Q Fe-S cyt c1 cyt c cyt a cyt a3 O2 cyt b succinate FAD Fe-S E'o (reduction potential)is a measure of how easily a compound can be reduced (how easily it can accept electron). All compounds are compared to reduction potential of hydrogen wich is 0.0 V. The larger the value of E'o of a carrier in ETC the better it functions as an electron acceptor (oxidizing factor). Electrons flow through the ETC components spontaneouslyin the direction of increasing reduction potentials. E'o of NADH= -0.32 volts (strong reducing agent)E'o of O2 = +0.82 volts (strong oxidizing agent)
Important characteristic of ETC is the amount of energy released upon electron transfer from one carrier to another. This energy can be calculated using the formula: Go’=-nFE’o n – number of electrons transferred from one carrier to another; F – the Faraday constant (23.06 kcal/volt mol); E’o– the difference in reduction potential between two carriers. When two electrons pass from NADH to O2 : Go’=-2*96,5*(+0,82-(-0,32)) = -52.6 kcal/mol