Proteins are the workhorses of the human body. Encoded by genes and assembled from chains of amino acids, they perform virtually every molecular task required for life — from catalysing chemical reactions and transporting molecules to providing structural support and defending against infection. The human proteome is estimated to contain between 20,000 and 25,000 distinct proteins, many of which exist in multiple forms due to post-translational modifications and alternative splicing.
All proteins are built from the same set of 20 standard amino acids linked together by peptide bonds. The sequence of amino acids — the primary structure — is determined by the corresponding gene. From this linear chain, regions fold into local patterns such as alpha-helices and beta-sheets (secondary structure), which in turn compact into a unique three-dimensional shape (tertiary structure). Some proteins consist of multiple folded chains held together to form a quaternary structure, as seen in haemoglobin's four subunits.
A protein's shape is not merely aesthetic: it dictates function. Enzymes rely on precisely shaped active sites to bind substrates; receptor proteins recognise specific signalling molecules; structural proteins interlock to form fibres and scaffolds. Misfolding, which can occur due to genetic mutation or environmental stress, often leads to loss of function or toxic aggregation — mechanisms central to diseases such as Alzheimer's and Parkinson's.
Enzymes are biological catalysts that accelerate chemical reactions without being consumed. Almost every metabolic reaction — from breaking down glucose to synthesising DNA — is driven by a specific enzyme. For example, DNA polymerase copies the genome during cell division, while amylase begins the digestion of carbohydrates in the mouth. The human body produces thousands of different enzymes, each highly specific to its target reaction.
Structural proteins provide mechanical support to cells and tissues. Collagen is the most abundant protein in the human body, forming the fibrous framework of skin, bone, cartilage, and tendons. Keratin gives strength to hair, nails, and the outer layers of skin. Inside cells, actin and tubulin assemble into dynamic filaments that maintain cell shape and enable movement.
Transport proteins carry molecules through the blood or across cell membranes. Haemoglobin, found in red blood cells, picks up oxygen in the lungs and delivers it to tissues throughout the body. Albumin, the most abundant protein in blood plasma, transports fatty acids, hormones, and drugs. Membrane-embedded transport proteins such as aquaporins and ion channels control the selective passage of water and ions into and out of cells.
Cell communication relies on signalling proteins. Insulin, a small protein hormone secreted by the pancreas, signals cells to take up glucose from the bloodstream. Growth factors such as EGF and VEGF direct cell proliferation and blood vessel formation. These signals are detected by receptor proteins embedded in the cell membrane; when activated, they trigger cascades of intracellular events that alter gene expression, metabolism, or cell behaviour.
The immune system depends heavily on specialised proteins. Antibodies (immunoglobulins) are Y-shaped proteins produced by B cells that recognise and bind to specific antigens on pathogens, marking them for destruction. Complement proteins form a cascading system that punches holes in bacterial membranes. Cytokines — including interleukins and interferons — act as chemical messengers that coordinate the behaviour of immune cells during infection and inflammation.
Motor proteins convert chemical energy into mechanical movement. Myosin interacts with actin filaments to produce the muscle contractions that allow movement and heartbeat. Kinesin and dynein act as molecular transporters, walking along microtubules to carry cargo — organelles, vesicles, and proteins — to specific locations within the cell. These motors are also essential for separating chromosomes during cell division.
| Protein | Class | Key Function | Location |
|---|---|---|---|
| Haemoglobin | Transport | Oxygen delivery | Red blood cells |
| Collagen | Structural | Tissue scaffolding | Skin, bone, tendons |
| Insulin | Hormone/Signalling | Blood glucose regulation | Pancreas / bloodstream |
| Albumin | Transport | Molecular carrier in blood | Blood plasma |
| Immunoglobulin G (IgG) | Immune | Pathogen recognition | Blood, lymph |
| Myosin | Motor | Muscle contraction | Muscle cells |
| DNA Polymerase | Enzyme | DNA replication | Cell nucleus |
| p53 | Regulatory/Tumour suppressor | DNA damage response, apoptosis | Nucleus |
| Keratin | Structural | Mechanical protection | Hair, nails, skin |
| Interferon-gamma | Cytokine/Immune | Antiviral and immune activation | Immune cells |
Proteins are produced through two sequential processes: transcription and translation. In transcription, a gene's DNA sequence is copied into messenger RNA (mRNA) in the cell nucleus. The mRNA then travels to ribosomes in the cytoplasm, where it is decoded codon by codon during translation, with transfer RNA (tRNA) molecules delivering the appropriate amino acid for each codon. The ribosome links these amino acids into a growing polypeptide chain.
Gene expression is tightly regulated at multiple levels. Transcription factors bind to DNA to switch genes on or off. MicroRNAs can silence mRNAs before they are translated. Post-translational modifications — such as phosphorylation, glycosylation, and ubiquitination — further fine-tune protein activity, stability, and localisation after synthesis.
Many diseases are fundamentally disorders of protein function. In sickle cell anaemia, a single amino acid change in haemoglobin causes red blood cells to deform under low oxygen conditions. In cystic fibrosis, mutations in the CFTR protein — a chloride ion channel — lead to thick mucus accumulation in the lungs. Cancer frequently involves proteins that regulate cell growth becoming stuck in an active state, or tumour suppressors such as p53 being inactivated.
Modern medicine exploits proteins extensively. Therapeutic proteins — including recombinant insulin, erythropoietin, and monoclonal antibodies — have transformed treatment for diabetes, anaemia, and cancer respectively. Protein-targeted drugs (such as kinase inhibitors) are designed to fit into the active sites of disease-causing proteins and block their activity. Advances in structural biology, particularly cryo-electron microscopy, have made it possible to visualise protein structures in unprecedented detail, dramatically accelerating drug discovery.
The proteome — the full complement of proteins expressed by an organism at a given time — is far more complex than the genome. While the genome is largely static, the proteome varies between cell types, developmental stages, and disease states. Proteomics, the large-scale study of proteins, uses techniques such as mass spectrometry to map these differences and identify potential disease biomarkers.
One landmark achievement in recent years has been the development of AlphaFold by DeepMind, an AI system that can predict protein structures from amino acid sequences with remarkable accuracy. This has effectively solved one of biology's grand challenges — the protein folding problem — and has opened new frontiers in understanding the molecular basis of disease and in designing novel therapeutic proteins from scratch.
This document provides a general scientific overview of human proteins for educational purposes.