Glucose Metabolism - PDF

Glycolysis Abdur Rahman Fate of Pyruvate ad Glucose https://www.wiley.com/college/pratt/0471393878/student/animations/citric_ acid_cycle/index.html Mechanism of Action of PDH Complex • • • A : pyruvate decarboxylase - thiamine pyrophosphate TPP removes COOH from pyruvate leaving 2 carbon...

Glycolysis Abdur Rahman Fate of Pyruvate ad Glucose https://www.wiley.com/college/pratt/0471393878/student/animations/citric_ acid_cycle/index.html Mechanism of Action of PDH Complex • • • A : pyruvate decarboxylase - thiamine pyrophosphate TPP removes COOH from pyruvate leaving 2 carbon fragment that binds the acyl fragment to the S-TPP. B : lipomide reductase transacetylase - lipoate 2 carbon acyl group is transferred to one lipoamide arm, and then to the other, to position it for CoASH transfer. C : dihydrolipoyl dehydrogenase- CoASH, FAD, NAD+ acyl group is transferred to CoASH; the reduced lipoamides transfers 2H's to E-FAD --> E-FADH2, and then E-FADH2 passes H to NAD+ --> NADH O O || || CH3-C-COOH -----> CH3-C-SCoA NAD -----------> NADH Regulation of PDH complex • Regulated by both allosteric and covalent modification • Allosteric regulation: – Inhibited by: ATP, Acetyl CoA, NADH (↑energy situation) – Activated by: AMP, CoA, NAD+ (↓energy situation) • Covalent modification: – Phosphorylation/dephosphorylation of E1 – Phospho-enzyme → inactive – Dephospho-enzyme → active Regulation of Pyruvate Dehydrogenase Complex Krebs Cycle Dr. Abdur Rahman non Regulation of TCA cycle i Regulated at 3 exergonic steps: 1. By substrate availability 2. Accumulation of product i 3. Allosteric feedback inhibition Energy yield in Krebs Cycle Total energy formation • Reaction 3 : 3 ATPs • Reaction 4: 3 ATPs • Reaction 5: 1 ATP (as GTP) • Reaction 6: 2 ATPs • Reaction 8: 3 ATPs • ----------------------• 12 ATPs (one mol of acetyl CoA) • x2 -----------------------24 ATPs per glucose (in Kreb’s cycle) 6 ATPs per glucose (pyruvate to acetyl CoA) 8 ATPs per glucose (Glycolytic pathways) ---------------------------------------38 ATPs (262.8 kcal) Harvesting energy from electron carriers The Electron Transport Chain Electron Transport System • • • • Glucose has C, H and O In glycolysis + Krebs cycle C and O released as CO2 H taken up by NAD+ and FAD → NADH and FADH2 High energy e- have to taken up by another acceptor • Travels through a series of electron acceptors and eventually end up with O2 • O2 get reduced; associate with H+ → H2O • Energy released during e- flow is used for ATP synthesis ATP synthesis • Two systems for ATP synthesis: – Substrate level phoshphorylation – Oxidative phosphorylation • Energy from high energy e- is used for oxidative phosphorylation • ETS is present on the inner mitochondrial membrane • With one exception (CoQ) all are proteins Electron Transport System • Arranged in 5 complexes: – Complex I (NAD dehydrogenase) – Complex II (succinate-Q reductase complex) – Complex III (cytochrome b) – Complex IV (cytochrome oxidase or cyt. a + a3) – Complex V (ATP synthase) L Harvesting energy • During electron transfer electron move from lower affinity carrier to a high affinity carrier (can accept electron in lower energy state) • Energy is released • This energy is utilized to pump H+ from the matrix into the inter membrane space • Electrical and pH gradient is established • H+ tend to dissipate back into matrix through ATP synthase • Energy of H+ flow is used for ADP + Pi → ATP ATP synthesis inhibitors • Oligomycin: – inhibit flow of H+ through ATP synthase • Cyanide, CO: – inhibit cytochrome oxidase • Barbiturates: – inhibit electron transfer to CoQ from Fe-S center • All of these inhibit ETC Un-couplers • ETC and ATP synthesis are coupled • If H+ flow is blocked, ETC stops • Uncoupler provide a leak for H+; ETC goes on but no ATP synthesis • H+ flow produce heat instead of ATP • Synthetic uncoupler: – 2,4-dinitrophenol • Uncoupling proteins: – Normally present – Thermogenin in brown fat in newborn babies Hexose mono phosphate shunt (HMP-shunt) or Pentose phosphate Pathway Dr. Abdur Rahman HMP-shunt • A pathway for glucose breakdown in which no ATP is produced or consumed • Purpose: 1. Produce NADPH (used for synthetic reactions) 2. Ribose-5-PO4 (nucleotide synthesis) 3. Provide mechanism for utilizing 5C and 7C sugars in the diet HMP-Shunt-importance • Important for: – Liver, mammary gland (fatty acid synthesis) – Adrenal cortex (steroid hormone synthesis) – RBC (to reduce glutathione) i HMP-Shunt Reactions • Two types of reactions: • Oxidative reactions: – 2 reactions to produce NADPH; are irreversible – Results in formation of Ribulose-5-PO4 (5C keto sugar) • Reversible reactions: – Generate Ribose-5-PO4 – Inter-conversion of 3, 4, 5, 6, and 7C sugars – Regenerate intermediates for glycolysis Non-oxidative part of pentose phosphate pathway: Uses of NADPH 1. Reductive Biosynthesis: – FA, cholesterol, steroid synthesis so 2. Reduction of Hydrogen peroxide: g – Regenerate reduced glutathione; H2O2 is byproduct of metabolism; has to be detoxified by glutathione peroxidase; glutathione is oxidized; has to be regenerated by NADPH I 3. Detoxification of drugs, carcinogen etc – Oxidized by cytochrome P450; uses O2 and NADPH; solubilize fort excretion Uses of NADPH 4. Oxidative burst by phagocytic cells to kill microbes – O2 converted into O2- (superoxide) by NADPH oxidase 5. Nitric oxide synthesis: – Arginine, O2 and NADPH are substrates for NO synthase – NO involved in vascular smooth muscle tone, in immunity, is a neurotransmitter Help the Cell! • A hypothetical cell has only glucose available for its needs • As a Biochemist guide the cell to adjust its Biochemical pathways to cope with the following situations: – – – – The cell needs ATP only The cell needs only NADPH (no ATP or Ribose-5-PO4) The cell needs only Ribose-5-PO4 (no NADPH or ATP) The cell needs NADPH and ATP but no Ribose-5-PO4 • Remember that cells do not make anything that they do not need