Protein

Proteins are large, complex molecules made of amino acids, the building blocks of all living organisms. They perform a vast array of functions in cells and tissues, including catalyzing chemical reactions, providing structural support, transporting molecules, signaling between cells, and defending against pathogens.

Proteins originate from sequences of amino acids linked by peptide bonds. The linear chain folds into a unique three-dimensional structure. The folding process creates four levels of organization: primary (amino acid sequence), secondary (alpha helices and beta sheets), tertiary (overall 3D shape of a single polypeptide), and quaternary (assembly of multiple polypeptides).

Biological synthesis: Genes encode proteins through transcription and translation on ribosomes. After synthesis, proteins may be modified and processed in the endoplasmic reticulum and Golgi apparatus.

Digestion and nutrition: In the digestive system, stomach acid and enzymes like pepsin begin protein breakdown into polypeptides and amino acids, which are further digested by pancreatic proteases in the small intestine and absorbed by enterocytes. The absorbed amino acids enter the bloodstream and are used for protein synthesis or energy.

Dietary aspects: Proteins come from animal and plant sources. Complete proteins supply all essential amino acids in sufficient amounts (common in animal products; many plant proteins can be combined to achieve completeness). The recommended intake for adults is about 0.8 grams per kilogram of body weight per day, though needs vary with age, activity, and health. Excessive intake may burden the kidneys in susceptible individuals.

Functions and roles: Enzymes accelerate reactions; structural proteins support cells and tissues; transport proteins carry substances; signaling and regulatory proteins control processes; antibodies defend against pathogens; hormones regulate physiology.