Protein Structure

Proteins are polymers of amino acids, also known as polypeptides, and are the most diverse type of biomolecule. They fulfill various roles, including enzymes, receptors, oxygen carriers like hemoglobin, and structural components of muscle and organ tissue.

Amino acids polymerize by forming peptide bonds, which are amide functional groups, through a dehydration reaction. The loss of a water molecule allows two amino acids to link. Two amino acids form a dipeptide, 3-10 form an oligopeptide, and more than 10 form a polypeptide. Proteins are large polypeptides of 300-1000 amino acids with biological activity.

Every peptide has an N-terminus (amino group end) and a C-terminus (carboxyl group end). Proteins are conventionally written with the N-terminus on the left. Each amino acid unit within a polypeptide is called a residue.

Primary structure is the linear sequence of amino acids in a polypeptide chain, without considering its three-dimensional shape. Each amino acid can be abbreviated with a three-letter or one-letter code. This sequence dictates how the protein will fold.

Secondary structure describes localized folding patterns of the polypeptide backbone, occurring over a few dozen amino acid residues. The peptide bond has partial pi bond character, making the backbone rigid, while R-group bonds are flexible. Proteins fold to achieve the lowest-energy conformation through electrostatic interactions. Common motifs include beta-pleated sheets and alpha-helices, where amide groups interact through hydrogen bonds.

Tertiary structure involves the further three-dimensional folding of the entire polypeptide chain, which is specific and crucial for the protein's function. Hydrophobic residues tend to be in the interior, away from aqueous solvent, while hydrophilic residues are on the surface to interact with water. Disulfide bonds, covalent linkages between cysteine thiol groups, also stabilize tertiary structure. Proteins are categorized as globular (highly folded and compact) or fibrous (long and spindly).

Quaternary structure applies to proteins composed of multiple polypeptide subunits. These subunits interact through strong electrostatic forces, arranging themselves in specific ways to form a larger functional protein, such as hemoglobin. Proteins with only one polypeptide chain do not have quaternary structure.

Even a minor change in the primary sequence can significantly alter the overall protein structure and function. For instance, in sickle cell disease, a single amino acid change in hemoglobin (glutamic acid to valine) leads to altered protein folding, causing red blood cells to become sickle-shaped and clog blood vessels. Understanding biomolecule structure and function is vital for comprehending health and disease.