Biochemistry is the branch of chemistry concerned with the chemical composition, molecular structure, reactions, interactions and transformations that occur in living organisms and biological systems. It investigates proteins, nucleic acids, carbohydrates, lipids, metabolites, cofactors, ions and numerous other molecular species and seeks to explain how their chemical properties produce biological structure and function. Biochemistry operates from the molecular scale of individual chemical bonds and reaction mechanisms to the network scale of metabolism and cellular regulation.
Its major subjects include protein structure and folding, enzyme catalysis, nucleic acid chemistry, membrane processes, metabolism, bioenergetics, molecular recognition and biochemical signalling. It overlaps with Molecular Biology, Cell Biology, Genetics, Pharmacology and Physiology, but places particular emphasis on chemical mechanisms. The subject asks how biological reactions occur under aqueous, relatively mild conditions; how information is chemically stored and transferred; how enzymes accelerate reactions; how energy is captured and transformed; and how thousands of reactions are coordinated within cells.
Proteins are among the most important biochemical macromolecules because their three dimensional structures enable an enormous range of biological functions. Proteins are polymers of amino acids linked by peptide bonds, and their amino acid sequences provide the primary structural information from which higher order organisation develops.
Secondary structures such as alpha helices and beta sheets arise largely through hydrogen bonding involving peptide backbone groups, while tertiary structure depends on hydrophobic interactions, hydrogen bonding, electrostatic interactions, van der Waals forces, disulfide bonds and other interactions. Some proteins associate into quaternary structures containing multiple polypeptide chains. Protein functions include catalysis, transport, molecular recognition, structural support, signalling, regulation and mechanical activity. Folding is influenced by solvent, temperature, pH, ionic conditions and cellular factors, and misfolding can impair biological function. Structural techniques including X ray crystallography, nuclear magnetic resonance spectroscopy and cryogenic electron microscopy provide complementary information about protein architecture and molecular interactions.
Enzymes are biological catalysts that accelerate chemical reactions by lowering the effective activation barrier through alternative reaction pathways. They do not alter the overall thermodynamic equilibrium of a reaction, but they can increase reaction rates enormously under biological conditions. Enzyme active sites create specific chemical environments that position substrates and stabilise transition states through interactions such as hydrogen bonding, electrostatic attraction, covalent catalysis and metal ion coordination.
Enzyme activity depends on substrate concentration, enzyme concentration, temperature, pH, inhibitors, activators and cofactors. Michaelis Menten kinetics provides a foundational model for many enzyme systems, introducing parameters describing apparent substrate affinity and maximum catalytic rate, although real enzymes may exhibit cooperativity, allostery, multiple substrates, conformational changes and complex regulation. Enzyme inhibition can be competitive, noncompetitive, uncompetitive or mixed under simplified kinetic classifications, but real biological inhibition can be more complicated. Enzymology therefore connects molecular structure, reaction mechanisms and cellular regulation.
Nucleic acid Biochemistry concerns the chemical structures and reactions of DNA and RNA. Nucleotides contain nitrogenous bases, pentose sugars and phosphate groups, and polymerisation produces phosphodiester linked chains. DNA commonly forms a double helical architecture in which complementary base pairing permits accurate information storage and replication. RNA can adopt diverse structures and functions, serving as messenger, adaptor, structural, catalytic and regulatory molecules.
DNA replication involves coordinated enzymatic processes that copy genetic information, while transcription converts DNA sequence information into RNA and translation uses ribosomes and transfer RNAs to produce proteins. Biochemistry also investigates nucleotide biosynthesis and degradation, DNA repair, RNA processing and chemical modification of nucleic acids. The chemical properties of nucleobases, sugars and phosphate groups determine stability, recognition and reactivity. Understanding these mechanisms provides the molecular foundation for genetics and biotechnology and explains how chemical information is maintained, copied and expressed in biological systems.
Metabolism is the interconnected collection of chemical reactions through which organisms obtain energy, construct cellular components, maintain internal organisation and dispose of waste products. Catabolic pathways generally degrade molecules and release usable energy or reducing equivalents, while anabolic pathways consume energy and precursor molecules to build larger biological structures. Glycolysis, the citric acid cycle, oxidative phosphorylation, fatty acid metabolism, amino acid metabolism and nucleotide metabolism are major examples, although organisms possess enormous additional metabolic diversity.
ATP acts as a major energy transfer molecule, while NADH and FADH2 carry reducing equivalents in many pathways. Metabolic reactions are organised into pathways because intermediates generated by one reaction can become substrates for another, allowing cells to coordinate carbon and energy flows. Regulation occurs through enzyme activity, allosteric interactions, covalent modification, substrate availability, compartmentalisation and signalling. Metabolism is consequently a dynamic network rather than a fixed sequence of independent reactions.
Bioenergetics applies thermodynamic principles to biological energy transformations. Organisms obtain usable energy from nutrients, light or inorganic chemical processes and convert it into forms capable of driving biosynthesis, transport, movement and other cellular work. Electron transfer reactions are central to many energy conserving processes. In aerobic respiration, electron transport systems transfer electrons through a series of carriers and use the resulting energy to establish an electrochemical gradient across a membrane. ATP synthase can then use the gradient to synthesise ATP.
Photosynthetic organisms use light driven electron transfer to generate chemical energy and reducing equivalents. The concept of free energy is essential because many biochemical reactions that are unfavourable in isolation can proceed when coupled to strongly favourable reactions. Proton motive force is a particularly important example of energy storage in an electrochemical gradient. Biochemical energy conversion therefore obeys general chemical and physical principles while using specialised molecular machinery to control the direction, timing and localisation of energy flow.
Biochemistry developed from the intersection of organic chemistry, physiological chemistry and biological investigation. Early studies of fermentation, digestion and metabolism demonstrated that living organisms perform chemically definable transformations. The isolation and characterisation of amino acids, sugars, lipids, enzymes, vitamins, hormones and nucleic acids progressively revealed the molecular basis of biological function. The discovery of DNA structure and the development of molecular genetics established powerful connections between biochemical reactions and hereditary information.
Modern Biochemistry now incorporates structural biology, proteomics, metabolomics, genomics, bioinformatics, systems biology and chemical biology. High sensitivity mass spectrometry, advanced microscopy and high throughput sequencing have enabled researchers to measure very large numbers of biological molecules and interactions. Despite this technological expansion, classical biochemical approaches involving purification, controlled reactions and mechanistic analysis remain important because large scale measurements still require chemical interpretation.
Biochemistry has major applications in medicine, biotechnology, agriculture, nutrition, pharmacology, diagnostics and industrial production. Clinical biochemistry measures molecules and activities in blood, urine and other biological samples to investigate physiological states and disease associated changes. Pharmaceutical research uses biochemical knowledge to identify molecular targets, understand drug mechanisms and investigate metabolism and toxicity. Industrial biotechnology uses enzymes and microorganisms to manufacture foods, chemicals, fuels and therapeutic products, while metabolic engineering modifies biochemical pathways to increase production of selected compounds.
Biochemistry also supports vaccine development, biomarker discovery, genetic engineering and molecular diagnostics. A major limitation is that biochemical behaviour observed in a purified system may differ substantially inside a living cell because cells contain compartmentalisation, molecular crowding, competing pathways, regulatory feedback and changing concentrations. Accurate biochemical understanding therefore requires connecting isolated molecular mechanisms with the broader biological environment in which those mechanisms operate.
