Chapter 9: Overview of Metabolism

• • • 9.1 Overview of Metabolism Metabolism is the collection of biochemical reactions in a free-living organism that converts chemical energy into work o Consists of catabolic and anabolic pathways Catabolic pathways lead to the degradation of macromolecules and nutrients for the purpose of energy capture (NADH and FADH2 formation) Anabolic pathways use energy available from ATP hydrolysis and the oxidation of reducing equivalents (e.g., NADPH) to synthesize biomolecules Flux of Metabolites • Refers to the rate at which substrates and products (metabolites) are interconverted • Determined by two primary factors: o Level of enzyme activity o Bioavailability of substrates 10 major metabolic pathways in plants and animals • Highly interdependent • Controlled by enzyme activity levels and substrate bioavailability Basic metabolic map emphasizing the major biomolecules • Many biomolecules and primary metabolites are shared Free energy is key • The overall free energy of a reaction (∆G’) should be spontaneous • If unfavorable, reactions may be couples o They may use shared intermediates o Some metabolic enzymes are components of large multiprotein complexes • • 9.2 Structures of Simple Sugars Monosaccharide: one sugar containing an aldehyde (aldoses) or a ketone (ketoses) o Example: Glucose, a polyhydroxyaldehyde o Example: Fructose, a polyhydroxyketone Taste sweet and are found naturally in fruits and vegetables Isomerization and simple sugars • In a Fischer projection, monosaccharides may be considered “L” or “D” depending on the location of the chiral carbon at the penultimate carbon Epimers • Two monosaccharides that differ in the -OH position around one chiral carbon Haworth projections • Simple sugars go through an intramolecular reaction to form either pyranose or furanose rings • This increases structural stability • The naming convention changes Reducing sugars • Simple sugars can be oxidized or reduced based on their functional groups • Reducing sugars: carbohydrates that react with oxidizing agents o Generally aldoses Disaccharides • Two monosaccharides linked together through an O-glycosidic bond • Formed through a condensation reaction • Three common disaccharides: o Lactose o Sucrose o Trehalose Lactase: A hydrolytic enzyme • Expressed in high levels in infants to aid in digestion of lactose • Declines in adults • • • • 9.3 Glycolysis Generates ATP under Anaerobic Conditions Glycolysis is the “splitting” of 1 molecule of glucose into 2 molecules of pyruvate Consists of 10 enzymatic reactions o Stage 1: ATP investment (reactions 1-5) o Stage 2: ATP production (reactions 6-10) Primary pathway for ATP generation under anaerobic conditions Metabolites are precursors for a large number of independent pathways, including mitochondrial ATP synthesis Free energy and glycolysis • This process involves multiple coupled reactions in order to be overall energetically favorable Glycolysis: Step-by-step • Reaction 1: hexokinase or glucokinase o Works via induced-fit mechanism o Phosphorylation reaction o ATP hydrolysis o Mg2+ required • Reaction 2: phosphoglucoisomerase o Isomerization • Reaction 3: phosphofructokinase-1 o Phosphorylation o ATP hydrolysis o Rate-limiting step of entire pathway • Reaction: aldolase • • • • • • o Cleavage o Uses Schiff base as an intermediate Reaction 5: triose phosphate isomerase o Isomerization o 2 DHAP are used in the subsequent steps of glycolysis o End of ATP investment stage Reaction 6: glyceraldehyde-3-phosphate dehydrogenase (GAPDH) o Phosphoryl group transfer o Produces 2 NADH Reaction 7: phosphoglycerate kinase o Substrate-level phosphorylation o Produces 2 ATP Reaction 8: phosphoglyceromutase o Phosphoryl shift Reaction 9: enolase o Dehydration Reaction 10: pyruvate kinase o Substrate-level phosphorylation o Produces 2 ATP Phosphofructosekinase-1 is regulated • Controls flux • AMP and ADP are indicators of low energy charge in the cell • ATP indicates high energy charge in the cell • Citrate is an inhibitor • • • • 9.4 Regulation of the Glycolytic Pathway The direction of metabolic flux depends on substrate availability 9.5 Metabolic Fate of Pyruvate Metabolized in one of three ways depending on organism and availability of oxygen Aerobic conditions: o ATP production (citrate cycle and electron transport chain) Anaerobic conditions: o Production of lactate o Ethanol production NAD+ regeneration • Required in order to maintain flux through GAPDH • Regenerated in the cytoplasm Lactate dehydrogenase deficiency • Enzyme is not functional • NAD+ levels are inadequate