OXIDATIVE PHOSPHORYLATION SUBSTRATE LEVEL PHOSPHORYLATION

Direct transfer of a phosphate group from a substrate to ADP for the formation of high energy ATP is known as substrate level phosphorylation. This reaction is mostly catalyzed by the enzyme kinases. Phosphate group donor directly donates or transfers a phosphate group to ADP without the involvement of an intermediate between the donor and ADP. 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 anaerobic conditions. The phosphate group is transferred from the first molecule and received by the second molecule. The energy released during the breakage of the phosphate group is used to phosphorylation of ADP in substrate level phosphorylation, and it is known as reaction coupling. Substrate Level Phosphorylation

Glycolysis is the commonest example where ATP is synthesized via substrate level phosphorylation when two phosphoenol pyruvate molecules are converted into two pyruvate molecules by pyruvate kinase enzyme under aerobic or anaerobic conditions. Moreover, during the Krebs cycle, ATPs are produced via substrate level phosphorylation Substrate Level Phosphorylation

1,3 Bisphosphoglycerate 3 phosphoglycerate

Phosphoenol Pyruvate Pyruvate 1913-1991

responsible for identifying and purifying Factor 1 (F1), the first part of the ATP synthase enzyme to be characterised. F1 is only a part of a larger ATP synthase complex known as Complex V. It is a peripheral membrane protein attached to component Fo, which is integral to the membrane. OXIDATIVE PHOSPHORYLATION

+ The NADH+H & FADH2 formed in glycolysis, fatty acid oxidation & Citric acid cycle are energy rich molecules because each contains a pair of electrons having high transfer potential. When these electrons are donated to molecular oxygen a large amount of free energy is liberated . Oxidative phosphorylation is the process in which ATP is formed as electrons are transferred from to oxygen through + NADH+H or FADH2 a series of electron carriers. • This is the major source of energy in aerobic organisms.

Inner and outer mitochondrial membranes enclose two spaces: the matrix and intermembrane space

(1) Outer membrane : Contains channel-forming protein, called Porin. Permeable to all molecules of 5000 daltons or less. (2) Inner membrane (Impermeability): Contains proteins with three types of functions: (a) Electron-transport chain: Carry out oxidation reactions; (b) ATP synthase: Makes ATP in the matrix; (c) Transport proteins: Allow the passage of metabolites (3) Intermembrane space: Contains several enzymes use ATP to phosphorylate other nucleotides. (4) Matrix: Enzymes; Mit DNA, Ribosomes, etc. Localization of metabolic functions within the mitochondrion (1) Outer membrane: Phospholipid synthesis; Fatty acid desaturation; Fatty acid elongation; (2) Intermembrane space: Nucleotide phosphorylation; (3) Inner membrane: Electron transport; Oxidative phosphorylation; Metabolite transport; (4) Matrix: Pyruvate oxidation;TCA cycle;? oxidation of fats; DNA replication; RNA transcription; Protein translation OXIDATIVE PHOSPHORYLATION IN EUKARYOTES TAKES PLACE IN MITOCHONDRIA Two membranes: Location of mitochondrial complexes outer membrane • Inner mitochondrial membrane: Electron transport chain inner membrane (folded into cristae) ATP synthase Two compartments: (1) the intermembrane space • Mitochondrial matrix: (2) the matrix Pyruvate dehydrogenase complex Citric acid cycle Fatty 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). Mitochondrial functions are localized in specific compartments

1. Outer membrane 4. Matrix • Fatty acid elongation – Pyruvate dehydrogenase • Fatty acid desaturation complex • Phospholipid synthesis – TCA cycle • Monoamine oxidase – Glutathione 2. Inner membrane dehydrogenase • Electron transport – Fatty acid oxidation • Oxidative – Urea cycle phosphorylation – DNA replication • Transport system – Transcription • Fatty acid transport – Translation 3. Intermembrane space • Creatine kinase • Adenylate kinase THE ELECTRON TRANSPORT CHAIN Series of enzyme complexes (electron carriers) embedded in the inner mitochondrial membrane, which

oxidize NADH +H+ and FADH2 and transport electrons to oxygen is called respiratory electron-transport chain (ETC). The sequence of electron carriers in ETC NADH FMN Fe-S Co-Q Fe-S cyt c cyt c cyt a cyt a O cyt b 1 3 2 succinate FAD Fe-S Oxidative phosphorylation

H+ transport results in an electrochemical gradient matrix Proton motive force: energy released by flow of H+ down its inter- gradient is used for ATP synthesis cristae membrane space ATP synthase: H+ channel that couples energy from H+ flow with inner outer ATP synthesis membrane mitochondrion membrane INHIBITORS OF OXIDATIVE PHOSPHORYLATION INHIBITORS OF OXIDATIVE PHOSPHORYLATION