Metabolism

Definition and Overview

Metabolism Defined

Metabolism: sum of all biochemical reactions in living cells. Purpose: convert nutrients into energy and biomolecules. Divided into two classes: catabolism (breakdown) and anabolism (synthesis). Integral for growth, reproduction, homeostasis.

Biochemical Scope

Includes enzymatic reactions transforming carbohydrates, lipids, proteins, nucleic acids. Involves energy transfer, molecular assembly, and waste elimination. Occurs in cytoplasm, mitochondria, chloroplasts.

Importance in Molecular Biology

Links gene expression to cellular function via metabolic enzymes. Provides precursors for nucleotides, amino acids, lipids. Regulates cell signaling and adaptation to environmental changes.

"Metabolism is the engine of life, driving cellular function through orchestrated chemical transformations." -- Albert Lehninger

Metabolic Pathways

Linear Pathways

Sequential chemical reactions converting substrates to final products. Example: glycolysis. Directional flow ensures efficient substrate utilization.

Cyclic Pathways

Reactions regenerating initial substrates. Example: citric acid cycle. Central role in energy extraction and metabolite synthesis.

Branched Pathways

Multiple routes diverging or converging to regulate metabolite flow. Enables flexibility and adaptation to cellular demands.

Role of Enzymes

Catalytic Function

Enzymes lower activation energy, increase reaction rates by 10^6–10^12 fold. Specificity: substrate binding via active sites. Mechanism: induced fit model.

Enzyme Classes

Six major classes: oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases. Each catalyzes distinct reaction types.

Allosteric Regulation

Effectors bind sites other than active site. Modulate enzyme activity, enable feedback inhibition or activation for metabolic control.

Energy Carriers: ATP and More

Adenosine Triphosphate (ATP)

Primary energy currency. High-energy phosphate bonds hydrolyzed to ADP + Pi release ~30.5 kJ/mol energy. Powers biosynthesis, transport, motility.

Nicotinamide Adenine Dinucleotide (NAD+/NADH)

Electron carrier in redox reactions. NAD+ accepts electrons, reduced to NADH. Couples catabolic oxidation to ATP synthesis.

Other Carriers

FAD/FADH2, Coenzyme A, UDP-glucose serve specialized roles in electron transport, acyl group transfer, carbohydrate metabolism.

Catabolism

Definition and Purpose

Breakdown of complex molecules into simpler units. Releases energy stored in chemical bonds. Provides ATP and reducing equivalents.

Major Catabolic Pathways

Glycolysis, beta-oxidation of fatty acids, proteolysis, citric acid cycle. Each generates intermediates feeding into energy production.

Energy Yield

Complete oxidation of glucose yields ~30-32 ATP molecules. Fatty acid oxidation yields more ATP per carbon. Efficiency depends on oxygen availability.

Glucose + 6 O2 → 6 CO2 + 6 H2O + Energy (ATP + heat)

Anabolism

Definition and Purpose

Synthesis of complex molecules from simpler precursors. Requires energy input, primarily from ATP hydrolysis. Supports growth, repair, storage.

Major Anabolic Processes

Protein synthesis, DNA replication, lipid synthesis, glycogen formation. Utilizes intermediates from catabolic pathways.

Energy Consumption

Consistent energy flow required. Example: peptide bond formation consumes 4 high-energy phosphate bonds per amino acid addition.

ATP + amino acid → aminoacyl-AMP + PPi (activation step)

Metabolic Regulation

Feedback Inhibition

End products inhibit enzymes at initial steps. Prevents overaccumulation and resource wastage. Example: ATP inhibits phosphofructokinase.

Allosteric Control

Effector molecules change enzyme conformation. Rapid response to metabolite concentration changes.

Hormonal Regulation

Insulin, glucagon, adrenaline modulate metabolism at systemic level. Coordinate fuel availability with demand.

Cellular Respiration

Overview

Process converting biochemical energy from nutrients into ATP. Includes glycolysis, citric acid cycle, oxidative phosphorylation.

Glycolysis

Occurs in cytoplasm. Converts glucose to pyruvate, net 2 ATP and 2 NADH produced. Anaerobic or aerobic conditions.

Oxidative Phosphorylation

Electron transport chain in mitochondria creates proton gradient. ATP synthase uses gradient to generate ATP. Oxygen is terminal electron acceptor.

Photosynthesis and Metabolism

Light-Dependent Reactions

Occurs in thylakoid membranes. Light energy converts ADP and NADP+ to ATP and NADPH. Oxygen produced by water splitting.

Calvin Cycle

Fixes CO2 into organic molecules using ATP and NADPH. Produces glyceraldehyde-3-phosphate, precursor for glucose and other carbohydrates.

Integration with Cellular Metabolism

Photosynthesis provides organic substrates for cellular respiration. Balances carbon fixation and energy demands in autotrophs.

Metabolic Disorders

Types of Disorders

Genetic or acquired defects in metabolic enzymes. Examples: phenylketonuria, diabetes mellitus, mitochondrial diseases.

Pathophysiology

Enzyme deficiency causes metabolite accumulation or deficiency. Disrupts energy balance, cellular function, organ systems.

Treatment Strategies

Dietary management, enzyme replacement, gene therapy under investigation. Symptom control and metabolic monitoring essential.

Experimental Techniques

Metabolomics

High-throughput analysis of metabolites using mass spectrometry, NMR. Provides comprehensive metabolic profiling.

Enzyme Kinetics

Measurement of reaction rates to determine enzyme properties (Km, Vmax). Essential for understanding metabolic control.

Isotope Tracing

Use of stable or radioactive isotopes to track metabolic flux. Reveals pathway activity and substrate utilization.

References

• Nelson, D. L., & Cox, M. M. Principles of Biochemistry. W. H. Freeman, 7th ed., 2017, pp. 550-620.
• Berg, J. M., Tymoczko, J. L., & Stryer, L. Biochemistry. W. H. Freeman, 8th ed., 2015, pp. 400-480.
• Voet, D., Voet, J. G., & Pratt, C. W. Fundamentals of Biochemistry: Life at the Molecular Level. Wiley, 5th ed., 2016, pp. 700-760.
• Alberts, B. et al. Molecular Biology of the Cell. Garland Science, 6th ed., 2014, pp. 680-750.
• Taiz, L., Zeiger, E., Møller, I. M., & Murphy, A. Plant Physiology and Development. Sinauer Associates, 6th ed., 2015, pp. 230-290.