Chapter 2: Protein Structure, Peptide Bonds & Function

Protein is a macromolecule built from amino acids — and both its shape and its job depend on how those amino acids are strung together and folded.

The amino acid, and the peptide bond

Every amino acid shares the same core layout: a central α-carbon bonded to an amino group (+NH3), a carboxyl group (-COO−), a hydrogen atom, and a variable R group (side chain) that gives each of the 20 standard amino acids its identity.

Amino acids link together by a peptide bond: the carboxyl group of one amino acid condenses with the amino group of the next, releasing one molecule of H2O. Chain length has its own vocabulary — dipeptide (two residues), tripeptide (three), and so on — and the number of peptide bonds is always one less than the number of amino acids in the chain.

Four levels of protein structure

Simple vs. complex proteins

• Simple proteins contain only amino acids.
• Complex (conjugated) proteins contain amino acids plus a non-protein component: heme in hemoglobin, lipid in lipoprotein, or carbohydrate in glycoprotein.

Seven functional categories of protein

Protein is not one job — it's a toolkit. The same amino-acid alphabet builds seven broad functional classes:

1. Regulator / bioregulator — hormones.
2. Biocatalyst — enzymes.
3. Transport — hemoglobin (O2), albumin (drugs, bilirubin, fatty acids), lipoprotein (lipids), transferrin (iron).
4. Contractile — actin and myosin, generating mechanical force in muscle.
5. Structural — collagen, tubulin, keratin, glycoproteins.
6. Protective / defense — immunoglobulins (antibodies), interferon, perforin, interleukins.
7. Receptor — membrane or intracellular proteins that bind a specific ligand. Receptors can work for the body (a hormone binding its receptor) or be hijacked against it — the clearest modern example being the ACE2 receptor's role in SARS-CoV-2 infection, covered in the next chapter.

Key teaching point

Structure and function are the same story told twice. A protein's primary sequence determines how it folds (secondary → tertiary → quaternary), and that final fold — not the sequence alone — determines which of the seven functional jobs above the protein can actually do. Denature the fold (heat, extreme pH, certain drugs) and the function is lost even though every peptide bond is still intact.