Concept 3.2 Proteins are Polymers with Important Structural and Metabolic Roles
Oligopeptides or peptides—short polymers of 20 or fewer amino acids (some hormones and
Polypeptides or proteins range in size from insulin, which has 51 amino acids, to huge
molecules such as the muscle protein titin, with 34,350 amino acids.
Amino acids are linked in condensation reactions to form peptide linkages or bonds.
Polymerization takes place in the amino to carboxyl direction.
There are four levels of protein structure:
o 1. Primary- held together by peptide bonds/ covalent bonds
o 2. Secondary- held by hydrogen bonds
o 3. tertiary
o 4. Quaternary
Secondary structure: regular, repeated spatial patterns in diﬀerent regions, resulting from
There are 2 major types of secondary structure:
o 1. α (alpha) helix—right-handed coil
o 2. β (beta) pleated sheet—two or more polypeptide chains are extended and aligned
Tertiary structure is the ﬁnal, folded, 3-D shape of a polypeptide.
Tertiary structure is determined by interactions between R groups, rather than interactions
between backbone constituents
Interactions between R groups determine tertiary structure.
o Disulﬁde bridges (covalent bond) hold a folded polypeptide together
o Hydrogen bonds stabilize folds
o Hydrophobic interactions side chains can aggregate
o van der Waals interactions between hydrophobic side chains
o Ionic interactions form salt bridges
o Two or more polypeptide chains (subunits) bind together by hydrophobic and ionic
interactions, and hydrogen bonds.
o These weak interactions allow small changes that aid in the protein’s f