Metabolism Overview

Metabolism Overview

• Intermediary metabolism involves biosynthesis, utilization, and degradation of low-molecular-weight compounds (intermediates)
• Metabolic energy can be derived from anaerobic environments, where molecular oxygen is not involved

Glycolysis

• A stage 2 pathway for degradation of carbohydrates, occurring in both aerobic and anaerobic cells
• Major input is glucose, derived from energy-storage polysaccharides or dietary carbohydrates
• Leads to pyruvate, a three-carbon a-keto acid
• In anaerobic organisms, pyruvate is reduced to various products, such as lactate or ethanol plus carbon dioxide (fermentations)

Oxidative Metabolism (Respiration)

• Pyruvate is oxidized to a metabolically activated two-carbon fragment, acetyl-CoA
• The citric acid cycle is the principal stage 3 pathway in aerobic organisms, accepting simple carbon compounds and oxidizing them to CO2
• Oxidative reactions in the citric acid cycle generate reduced electron carriers, driving ATP biosynthesis through electron transport and oxidative phosphorylation

Compartmentation and Allosteric Control

• Prevents futile cycles, which waste energy
• Regulation of anabolic and catabolic processes occurs through compartmentation and allosteric control

Biochemical Reaction Types

• Much of biological chemistry involves the carbonyl group, found in most biological molecules
• Redox chemistry lies at the core of metabolism, involving reversible electron transfer from a donor (reductant) to an acceptor (oxidant)

Energy Production

• Most biological energy is derived from the oxidation of fuel molecules, such as glucose
• Oxidation-reduction (redox) chemistry is essential for energy production

Anaerobic Metabolism

• Many microorganisms can or must live anaerobically, using substances other than oxygen as terminal electron acceptors
• Anaerobic metabolism involves the oxidation of reduced metabolites, with oxygen as the final electron acceptor

Carbohydrate Metabolism

• Catabolic and anabolic processes in anaerobic carbohydrate metabolism involve glycolysis, gluconeogenesis, and glycogen metabolism
• Glycolysis generates ATP anaerobically and provides fuel for aerobic energy-generating pathways

Glycolysis: An Overview

• The two phases of glycolysis are:
  1. Addition of phosphoryl groups to glucose, yielding compounds with low phosphate group-transfer potential
  2. Conversion of these low phosphate group-transfer potential intermediates into compounds with high phosphate group-transfer potential
  3. Chemical coupling of the energy-yielding hydrolysis of these high phosphate group-transfer potential compounds to the synthesis of ATP

Reactions of Glycolysis

• ATP is synthesized via substrate-level phosphorylation
• Glycolysis produces ATP via substrate-level phosphorylation

Energy and Electron Balance Sheets

• Glycolysis, which yields 2 ATP per glucose, is fast but releases only a small fraction of the energy available from glucose
• Energy profile of anaerobic glycolysis shows that most reactions function at or near equilibrium and are freely reversible in vivo

Gluconeogenesis

• Synthesis of glucose from noncarbohydrate precursors is essential for maintaining blood glucose levels within acceptable limits
• Liver and kidney cortex are the primary gluconeogenic tissues
• Gluconeogenesis uses specific enzymes to bypass three irreversible reactions of glycolysis

Polysaccharide Metabolism

• In animal metabolism, two primary sources of glucose are derived from polysaccharides:
  1. Digestion of dietary polysaccharides, chiefly starch from plant foodstuffs and glycogen from meat
  2. Mobilization of the animal's own glycogen reserves
• Breakdown of glycogen stores into usable energy involves sequential phosphorolytic cleavages of bonds, catalyzed by glycogen phosphorylase