Biochemistry 10 Carbohydrate Metabolism I – Aerobic Respiration

Biochemistry 10: Carbohydrate Metabolism I – Aerobic Respiratio Acetyl-CoA Acetyl-CoA: Contains a high-energy thioester bond that can be used to drive other reactions when hydrolysis occurs. Acetyl-CoA Formation: Can be formed from fatty acids, which enter the mitochondria using carriers. The fatty acid couples with CoA in the cytosol to form fatty acyl-CoA, which moves to the intermembrane space. The acyl (fatty acid) group is transferred to carnitine to form acyl-carnitine, which crosses the inner membrane. The acyl group is transferred to a mitochondrial CoA to re -form fatty acylCoA, which can undergo b-oxidation to form acetyl-CoA. Can also be formed from the carbon skeletons of ketogenic amino acids, ketone bodies, and alcohol. Pyruvate Dehydrogenase Oxidizes pyruvate, creating CO2; it requires thiamine (PDH): pyrophosphate (vitamin B1, TPP) and Mg2+. Dihydrolipoyl Oxidizes the remaining two-carbon molecule using lipoic Transacetylase: acid, and transfers the resulting acetyl group to CoA, forming acetyl-CoA. Dihydrolipoyl Uses FAD to reoxidize lipoic acid, forming FADH2. This Dehydrogenase: FADH2 can later transfer electrons to NAD+, forming NADH that can feed into the electron transport chain. Pyruvate Dehydrogenase Phosphorylates PDH when ATP or acetyl-CoA levels are Kinase: high, turning it off. Pyruvate Dehydrogenase Dephosphorylates PDH when ADP levels are high, turning Phosphatase: it on. Reactions of the Citric Acid Cycle * See appendix for full diagram Citric Acid Takes place in the mitochondrial matrix. Its main purpose is to Cycle: oxidize carbons in intermediates to CO2 and generate high-energy electron carriers (NADH and FADH2) and GTP. Citrate Couples acetyl-CoA to oxaloacetate and then hydrolyzes the Synthase: resulting product, forming citrate and CoA-SH. This enzyme is regulated by negative feedback from ATP, NADH, succinyl-CoA and citrate. Aconitase: Isomerizes citrate to isocitrate. Isocitrate Oxidizes and decarboxylates isocitrate to form a -ketoglutarate. Dehydrogenase: This enzyme generates the first CO2 and first NADH of the cycle. As the rate-limiting step of the citric acid cycle, it is heavily regulated: ATP and NADH are inhibitors; ADP and NAD+ are activators. a -Ketoglutarate Acts similarly to PDH complex, metabolizing a-ketoglutarate to Dehydrogenase form succinyl-CoA. This enzyme generates the second CO2 and Complex: second NADH of the cycle. It is inhibited by ATP, NADH, and succinyl-CoA; it is activated by ADP and Ca2+. Succinyl-CoAHydrolyzes the thioester bond in succinyl-CoA to form succinate Synthesis: and CoA-SH. This enzyme generates the one GTP generated in the cycle. Succinate Oxidizes succinate to form fumarate. This flavoprotein is anchored Dehydrogenase: to the inner mitochondrial membrane because it requires FAD, which is reduced to form the one FADH2 generated in the cycle. Fumarase: Hydrolyzes the alkene bond of fumarate, forming malate. Malate Oxidizes malate to oxaloacetate. This enzyme generates the third Dehydrogenase: and final NADH of the cycle. The Electron Transport Chain Acyl Carnitine Translocase: Mechanism for Acyl CoA to enter the mitochondrial matrix. The mitochondrial matrix is where Acyl CoA can undergo b-oxidation to form Acetyl-CoA. * See appendix for full diagram Electron Transport Takes place on the matrix-facing surface of the inner Chain: mitochondrial membrane. NADH donates electrons to the chain, which are passed from one complex to the next. As the ETC progresses, reduction potentials increase until oxygen, which has the highest reduction potential, receives the electrons. Complex I: NADH-CoQ Oxidoreductase. Uses an iron-sulfur cluster to transfer electrons from NADH to flavin mononucleotide (FMN), and then to CoQ, forming CoQH2. 4 H+ ions are translocated by Complex I. Complex II:Succinate-CoQ Oxidoreductase. Uses an iron-sulfur cluster to transfer electrons from succinate to FAD, and then to CoQ, forming CoQH2. No H+ pumping occurs at complex II. Oxidative Phosphorylation Proton-Motive The electrochemical gradient generated by the electron transport Force: chain across the inner mitochondrial membrane. The intermembrane space has a higher concentration of protons than the matrix; this gradient stores energy, which can be used to form ATP via chemiosmotic coupling. ATP Synthase: The enzyme responsible for generating ATP from ADP and Pi F0 Portion: An ion channel, allowing H+ to flow down the gradient from the intermembrane space to the matrix F1 Portion: Uses the energy released by the gradient to phosphorylate ADP into ATP. Complex III: CoQH2-Cytochrome C Oxidoreductase. Uses an iron-sulfur cluster to transfer elcetrons form CoQH2 to heme, forming cytochrome C as part of the Q cycle. 4 H+ ions are translocated by complex III. Complex IV: Cytochrome C Oxidase. Uses cytochromes and Cu2+ to transfer electrons in the form of hydride ions (H-) from cytochrome c to oxygen, forming water. 2 H+ ions are translocated by complex IV. NADH cannot cross the inner mitochondrial membrane. Therefore, one of two available shuttle mechanisms to transfer electrons in the mitochondrial matrix must be used. Glycerol 3-Phosphate Shuttle: Malate-Aspartate: Electrons are transferred from NADH to DHAP, forming glycerol 3-phosphate. These electrons can then be transferred to mitochondrial FAD, forming FADH2. Electrons are transferred from NADH to oxaloacetate, forming malate. Malate can then cross the inner mitochondrial membrane and transfer the electron to mitochondrial NAD+, forming NADH.