Exhaustive Study Notes on NADH Shuttling, Glycolysis, Beta-Oxidation, and Ketogenesis

Maltate-Aspartate Shuttle Mechanics: * In this shuttle system, aspartate is exported to the cytoplasm in exchange for glutamate entering the mitochondrial matrix. * When maltate enters the mitochondrial matrix, it is oxidized to produce Oxaloacetate (OAA) and NADH. * The NADH produced in the matrix can be processed by oxidative phosphorylation to yield a total of $2.5\,ATPs$ . * The transport of maltate into the mitochondrial matrix occurs in exchange for the export of alpha-ketoglutarate. * The convertibility between OAA + glutamate and aspartate + alpha-ketoglutarate within the mitochondrial matrix is catalyzed by the enzyme ASPT (Aspartate Aminotransferase).
Glycerol-3-Phosphate Shuttle Mechanics: * The oxidation of each glycerol-3-phosphate at the mitochondrial inner membrane results in the production of $1.5\,ATPs$ . * A protein integral to the mitochondrial inner membrane is responsible for oxidizing glycerol-3-phosphate to produce NAD+ during physical exertion in white / oxidative muscle cells. * The mitochondrial inner membrane glycerol-3-phosphate dehydrogenase reduces FAD during this process.

Glyceraldehyde 3-Phosphate Dehydrogenase (GAPDH): * GAPDH is identified as the primary redox enzyme of glycolysis. * During activity in the heart muscle and liver, GAPDH is utilized to reduce OAA (Oxaloacetate), which has been generated by cytoplasmic malate dehydrogenase. * The crucial limiting substrate for GAPDH that determines the overall rate of glycolysis when ATP levels are low is NAD+.
Dihydroxyacetone Phosphate (DHAP): * Glycerol-3-phosphate is produced through the reduction of DHAP.
Lactate Fermentation: * During small lactate fermentation, the creation of NAD+ occurs by reducing pyruvate.

Adenosine Monophosphate (AMP) Regulation: * AMP is the product of adenylate kinase when ATP levels are low. * AMP acts as an activator for the entry of glucose into muscle cells. * AMP binds to an allosteric binding site on PFK1 (Phosphofructokinase-1) to facilitate the formation of its active tetramer form.
Vesicular Transport: * When AMP levels are high, Rab & a (Rab proteins and associated guides) guide the assembly and movement of vesicles containing fatty acid translocase.

Preparation and Transport: * Fatty acids entering the cytoplasm are modified by acyl-CoA synthetase to prepare them for utilization in beta oxidation. * Malonyl CoA, a substrate for fatty acid synthase, inhibits the movement of acyl-chains into the mitochondrial matrix. * The mitochondrial inner membrane carnitine translocase is responsible for transferring acyl-chains into the matrix, where they are subsequently reattached to CoAs.
Step-by-Step Oxidation Process: * First Oxidation Step: The oxidation of acyl-CoAs by acyl CoA dehydrogenase in the matrix is linked to the reduction of FAD. * Product of First Oxidation: The oxidized product of acyl-CoA dehydrogenase is 2-enoyl-CoA. * Hydration Step: To prepare for the second oxidation, 2-enoyl-CoA must react with water ( $H_2O$ ) to produce 3-hydroxyacyl-CoA. * Second Oxidation Step: The oxidation of 3-hydroxyacyl-CoA is coupled to the reduction of NAD+. * Enzyme and Product of Second Oxidation: The product of 3-hydroxyacyl dehydrogenase (BCDH) is 3-ketoacyl-CoA. * Thiolysis Step: The enzyme thiolase reacts 3-ketoacyl-CoAs with CoA at the beta-keto group.
Specific Yields and Scenarios: * At the conclusion of one round of beta oxidation of stearoyl-CoA, thiolase produces a palmitoyl-CoA and one acetyl-CoA.

Ketogenic Pathways: * When pyruvate levels are low within the mitochondrial matrix during beta oxidation, thiolase converts acetyl-CoAs into acetoacetyl-CoA. * The $6$ -carbon-CoA intermediate produced during the process of ketogenesis is HMG-CoA.
Ketone Bodies and CNS Survival: * Beta-hydroxybutyrate is the enzymatically reduced product of ketogenesis utilized by the Central Nervous System (CNS) to survive during periods of hypoglycemia. * The $3$ -carbon waste product generated during ketogenesis is acetone.

Question: What acts as the limiting substrate for GAPDH when ATP is low?
Response: NAD+ is the crucial limiting substrate that sets the rate of glycolysis in these conditions.
Question: How does the CNS survive hypoglycemia?
Response: It utilizes beta-hydroxybutyrate, which is the enzymatically reduced product of ketogenesis.
Question: What enzyme converts acetyl-CoAs when pyruvate is low?
Response: Thiolase converts them to acetoacetyl-CoA.
Question: What is the specific exchange in the maltate-aspartate shuttle for matrix entry?
Response: Aspartate is exported to the cytoplasm in exchange for glutamate entering the matrix.