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Protein structure has four hierarchical levels. Primary structure is the linear sequence of amino acids read from the N-terminal to the C-terminal. Secondary structure forms through hydrogen bonding, creating alpha helices and beta-pleated sheets. Tertiary structure is the three-dimensional shape resulting from interactions between amino acid R-groups. Quaternary structure occurs when multiple polypeptide chains assemble together, as in hemoglobin's four subunits.

Amino acids are connected by peptide bonds, which form between the carboxyl group of one amino acid and the amino group of the next. This creates a linear polymer chain with a free amino end (N-terminal) and a free carboxyl end (C-terminal). The resulting polypeptide chain can then fold into secondary and tertiary structures through various molecular interactions.

Secondary structures form through hydrogen bonding between the carbonyl oxygen of one amino acid and the amino hydrogen of another amino acid in the polypeptide backbone. In alpha helices, these bonds occur between amino acids spaced four residues apart, creating a spiral structure with 3.6 residues per turn. In beta-pleated sheets, hydrogen bonds form between adjacent polypeptide chains aligned parallel or antiparallel to each other.

Tertiary structure results from interactions between amino acid R-groups. Ionic bonds form between R-groups with opposite charges, while hydrophobic interactions cluster nonpolar R-groups in the protein's interior. Disulfide bridges form covalent bonds between cysteine residues. Hydrogen bonds between polar R-groups also contribute. These combined weak and strong interactions fold the polypeptide into its final three-dimensional shape.

Protein location depends on the distribution of hydrophilic and hydrophobic amino acids. Cytoplasmic proteins have hydrophilic amino acids on their surfaces to interact with water and hydrophobic cores inside. Membrane proteins have the opposite arrangement: hydrophobic amino acids on the surface interact with glycerophospholipids in the membrane, while hydrophilic amino acids form the core. This arrangement ensures proper protein positioning and function.

Protein function depends critically on three-dimensional structure. When a protein loses its shape, it becomes nonfunctional. For example, in sickle cell anemia, a single amino acid substitution in hemoglobin's beta chain causes the protein to form long fibers, distorting red blood cells into a sickle shape. This structural change prevents normal oxygen transport and causes serious health complications including breathlessness, dizziness, and pain.

Quaternary structure forms when two or more polypeptide chains assemble into a single functional protein complex. Weak interactions like hydrogen bonds and disulfide linkages stabilize these multi-subunit assemblies. Hemoglobin exemplifies quaternary structure with four subunits: two alpha and two beta chains. Insulin also exhibits quaternary structure through disulfide bonds linking its separate polypeptide chains after post-translational modification.