A protein is a biological polymer composed of one or more chains of amino acids. Proteins are essential components of cells, providing structural support and performing much of their chemical and regulatory work. Their properties depend on amino-acid sequence, three-dimensional organization, and interactions with other molecules. Although many proteins adopt relatively stable folded structures, others contain flexible regions that remain functional without a single fixed shape. (genome.gov)
Chemical composition
Amino acids in a protein chain are connected by covalent peptide bonds, forming a polypeptide. Each amino acid contributes to a repeating backbone and carries a side chain whose chemical properties influence the protein’s behavior. Side chains may be nonpolar, polar, acidic, or basic. Their arrangement affects solubility, molecular recognition, and the reactions a protein can perform. Proteins principally contain carbon, hydrogen, oxygen, and nitrogen; sulfur occurs in the amino acids cysteine and methionine. (ncbi.nlm.nih.gov)
Most proteins are assembled using a standard set of 20 amino acids. A chain has directionality, extending from an amino, or N, terminus to a carboxyl, or C, terminus. “Peptide” generally describes a shorter amino-acid chain, while “protein” commonly denotes a larger biological product; there is no universally applicable length boundary between them. Some functional proteins consist of several polypeptide subunits rather than a single chain. (ncbi.nlm.nih.gov)
Synthesis and turnover
The sequence of a protein is encoded by a protein-coding gene in DNA. During transcription, genetic information is copied into messenger RNA. In translation, a ribosome reads the RNA sequence according to the genetic code, while transfer RNAs deliver the corresponding amino acids. The growing chain is synthesized from its N terminus toward its C terminus. (genome.gov)
A newly synthesized chain may require further processing before becoming functional. Proteins can be cleaved, joined to other subunits, or chemically modified through processes such as phosphorylation and attachment of sugar groups. Molecular chaperones assist many proteins in folding and help prevent inappropriate aggregation. Cells also continually degrade proteins: the ubiquitin–proteasome system selectively removes many intracellular proteins, allowing their constituent amino acids to be recycled. Protein abundance therefore reflects both production and breakdown. (ncbi.nlm.nih.gov)
Structure and folding
Protein structure is conventionally described at four levels:
- Primary structure: the amino-acid sequence.
- Secondary structure: local backbone arrangements, especially the alpha helix and beta sheet.
- Tertiary structure: the overall three-dimensional arrangement of a single polypeptide.
- Quaternary structure: the organization of multiple polypeptide subunits into an assembly.
Secondary structures are stabilized largely by backbone hydrogen bonds. Overall folding also involves electrostatic interactions, van der Waals forces, and sometimes covalent disulfide bonds. The hydrophobic effect commonly favors burial of nonpolar side chains inside soluble proteins. (ncbi.nlm.nih.gov)
Protein folding depends on sequence and environmental conditions. Heat, extreme acidity or alkalinity, and certain chemicals can disrupt a protein’s organization, causing denaturation and often loss of activity without necessarily breaking its backbone. Some proteins regain their structure when favorable conditions return; others aggregate irreversibly. Proteins are also dynamic: intrinsically disordered regions sample multiple conformations, and some acquire a more defined structure when they bind a partner. (ncbi.nlm.nih.gov)
Biological functions
Many proteins are enzymes, accelerating chemical reactions through catalysis. Their active sites position reacting molecules and facilitate chemical transformations. Some require associated metal ions or small organic cofactors. Enzyme activity can be regulated by binding other molecules or by chemical modification, connecting individual reactions to wider cellular control systems. (ncbi.nlm.nih.gov)
Other proteins perform structural, transport, signaling, and defensive functions. Collagen supports connective tissues, while actin and tubulin contribute to cellular architecture and movement. Hemoglobin transports oxygen in blood. Insulin is a protein hormone involved in metabolic regulation, and antibodies recognize specific molecular targets in immune defense. Proteins embedded in the cell membrane serve as channels, transporters, and receptors, controlling exchanges and communication between cells and their surroundings. (ncbi.nlm.nih.gov)
Dietary role
In human nutrition, dietary proteins supply amino acids used to synthesize the body’s own proteins. During digestion, enzymes break food proteins into smaller peptides and amino acids that can be absorbed. Nine amino acids are classified as essential for humans because the body cannot synthesize them in sufficient amounts; others can normally be produced internally, although some become conditionally essential under particular physiological circumstances. (medlineplus.gov)
Protein-containing foods include meat, fish, eggs, dairy products, legumes, nuts, and grains. Their amino-acid compositions vary, so the nutritional contribution of a protein depends on both its composition and availability to the body. Dietary protein is distinct from an intact functional protein in a cell: absorbed amino acids enter metabolic pathways rather than simply being incorporated as unchanged food proteins. (medlineplus.gov)
Study and structure prediction
Researchers characterize proteins through purification, biochemical assays, and methods such as mass spectrometry. Three-dimensional structures can be investigated using X-ray crystallography, nuclear magnetic resonance, and cryo-electron microscopy. These techniques provide different kinds of experimental evidence from which structural models are constructed. Experimentally determined structures are archived in the Protein Data Bank. (ncbi.nlm.nih.gov)
Computational systems such as AlphaFold predict protein structures using sequence information and patterns learned from structural data. Predicted models complement experimental research, but confidence varies across proteins and regions. A predicted shape alone does not establish biochemical function, reveal every biologically relevant conformation, or explain the complete folding pathway. (pdb101-west.rcsb.org)