Define protein
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Here is a thorough definition of protein, drawn from authoritative biochemistry textbooks:
Protein - Definition
A protein is a macromolecule composed of amino acids joined together in a linear sequence by peptide bonds (amide linkages between the α-carboxyl group of one amino acid and the α-amino group of the next). This chain folds into a unique three-dimensional shape that determines the protein's function.
- Lippincott Illustrated Reviews: Biochemistry, 8th ed. defines it directly: "Proteins are composed of amino acids that are joined together by peptide bonds in a linear sequence, and then folded into a unique three-dimensional shape that determines function."
When 50 or more amino acids are linked through peptide bonds, the result is an unbranched chain called a polypeptide or protein. Each amino acid in the chain is called a residue.
Levels of Protein Structure
Protein complexity is organized into four hierarchical levels:
| Level | Description |
|---|---|
| Primary | The linear sequence of amino acids; determined by the gene. Mutations here can cause disease (e.g., sickle cell anemia: valine substituted for glutamate at position 6 of β-globin). |
| Secondary | Local folding patterns stabilized by hydrogen bonds - primarily α-helices and β-sheets. |
| Tertiary | The overall three-dimensional shape of the full polypeptide chain. |
| Quaternary | The arrangement of multiple polypeptide subunits into one functional complex (e.g., hemoglobin). |
Key Properties
- Peptide bonds are resistant to heat and high concentrations of urea; they require prolonged exposure to strong acid or base at elevated temperatures to break nonenzymatically.
- The free N-terminal end is written to the left by convention; the free C-terminal end is to the right. Amino acid sequences are always read N- to C-terminal.
- The primary structure dictates conformation. Invariant regions (especially binding sites) must remain unchanged; variant regions tolerate some amino acid substitution.
- Conservative substitutions (replacing one amino acid with one of similar structure) in noncritical regions are generally tolerated and do not disrupt function.
Functions of Proteins
Proteins carry out virtually every biological task in the body, including:
- Enzymatic catalysis (enzymes)
- Structural support (collagen, keratin)
- Transport (hemoglobin carries O₂; albumin carries fatty acids and drugs)
- Signaling and regulation (hormone receptors, transcription factors)
- Immune defense (antibodies/immunoglobulins)
- Movement (actin, myosin in muscle)
- Gene expression regulation (histones, DNA-binding proteins)
Sources: Lippincott Illustrated Reviews: Biochemistry, 8th ed., Chapter 2 (Protein Structure); Basic Medical Biochemistry: A Clinical Approach, 6th ed., Chapter on Protein Primary Structure.
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