nounBeginner
/ˈproʊ.tiːn/
Greek πρωτεῖος (proteios) “of the first quality” · Coined by Berzelius, 1838
Simple Definition
A protein is a large macromolecule consisting of 50 or more amino acids linked by peptide bonds, folded into a specific three-dimensional structure that determines its biological function. Proteins serve as enzymes, structural components, antibodies, transporters, and signaling molecules.

Detailed Explanation

Proteins are the molecular workhorses of life, making up about 15–20% of human body mass. What distinguishes proteins from peptides is primarily size and structural complexity. Peptides (2–50 amino acids) are mostly flexible chains, while proteins (50+ amino acids) fold into precise three-dimensional shapes.

Protein structure exists at four levels: primary (amino acid sequence), secondary (alpha-helices and beta-sheets from backbone H-bonds), tertiary (overall 3D fold from R-group interactions), and quaternary (multi-subunit assemblies like hemoglobin).

Major protein types include enzymes (biological catalysts), structural proteins (collagen, keratin), transport proteins (hemoglobin, albumin), antibodies (immune defense), receptor proteins (cell signaling), and motor proteins (myosin, kinesin).

Proteins can be denatured — unfolded by heat, pH changes, or chemicals — losing their function. This is why cooking an egg turns the clear white opaque: heat denatures the albumin proteins. Peptides, lacking stable 3D folds, cannot be denatured.

Key Facts

  • Size: 50+ amino acids; can be thousands of residues long
  • ~20,000 protein-coding genes in the human genome
  • The body contains ~100,000+ distinct proteins
  • One of the largest known proteins: titin (~34,350 amino acids, found in muscle)
  • Proteins make up 15–20% of human body mass
  • 4 levels of structure: primary, secondary, tertiary, quaternary
  • Can be denatured (unfolded) by heat, pH, or chemicals

Biological Function

Proteins perform nearly every function in every cell. Enzymes catalyze biochemical reactions. Structural proteins (collagen, keratin) provide mechanical support. Transport proteins (hemoglobin) carry molecules. Antibodies recognize pathogens. Receptor proteins receive hormonal signals. Motor proteins generate movement in muscles and cells.

Example in Context

Scientific“Hemoglobin is a tetrameric protein with quaternary structure — four subunits cooperate to bind oxygen in the lungs and release it in peripheral tissues.”
Everyday“When you cook an egg, heat denatures the egg white proteins — unfolding their 3D structure, which is why the clear liquid turns white and solid.”

Frequently Asked Questions

A protein is a large molecule made of 50 or more amino acids linked by peptide bonds, folded into a specific 3D shape. Proteins function as enzymes, structural components, antibodies, and transport molecules.
Peptides have 2 to 50 amino acids and are generally flexible. Proteins have 50+ amino acids and fold into precise 3D structures. Proteins can be denatured by heat; peptides cannot.
Related Terms Peptide Amino Acid Peptide Bond Collagen

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