1.5 Structure & Function of Biological Macromolecules

Structure determines function

A macromolecule's shape decides what it can do. Proteins are the clearest example: they are polymers of amino acids joined by peptide bonds. There are 20 amino acids, and each has a different R-group (side chain) that gives it unique chemical properties.

The four levels of protein structure

Primary structure is the sequence of amino acids. Secondary structure is local folding into α-helices and β-pleated sheets, held by hydrogen bonds along the backbone. Tertiary structure is the overall 3-D shape of one polypeptide, driven by R-group interactions. Quaternary structure is two or more polypeptide chains together (e.g., hemoglobin's four subunits).

Shape can be lost

Because function depends on shape, changes in temperature or pH can denature a protein — unfolding it and destroying its function. The primary sequence ultimately dictates how the protein folds, so even a single wrong amino acid can change everything (as in sickle-cell hemoglobin).

Key terms

Amino acid

The monomer of proteins: a central carbon bonded to an amino group, a carboxyl group, an H, and a variable R-group.

Peptide bond

The covalent bond that links amino acids in a protein.

Primary structure

The sequence of amino acids in a polypeptide.

Tertiary structure

The overall 3-D shape of one polypeptide, from R-group interactions.

Denaturation

Loss of a protein's shape (and function) due to heat or pH changes.

Try an AP-style question

In sickle cell disease, a single amino acid substitution (glutamic acid → valine) changes hemoglobin's function. Which level of structure is changed FIRST?

1. Secondary structure
2. Tertiary structure
3. Primary structure
4. Quaternary structure