Protein. A protein is a polymer in which the monomer units are amino acids.

a-Amino acid. An a-amino acid is an amino acid in which the amino group and carboxyl group are attached to the same carbon atom.

Standard amino acid. A standard amino acid is one of the 20 a?amino acids that are normally present in protein.

Amino acid classifications. Amino acids are classified as nonpolar, polar neutral, polar basic, or polar acidic, depending on the nature of the side chain (R group) present.

Chirality of amino acids. Amino acids found in proteins are always left-handed (L isomer).

Zwitterion. A zwitterion is a molecule that has a positive charge on one atom and a negative charge on another atom. a-Amino acids exist as zwitterions because of an internal acid - base reaction between the amino and carboxyl groups present.

Peptides and peptide bonds. A peptide is a sequence of amino acids in which the amino acids are joined together through peptide bonds. A peptide bond is the bond between the carboxyl group of one amino acid and the amino group of another amino acid.

Protein primary structure. The primary structure of a protein is the sequence of amino acids present in the peptide chain or chains of the protein.

Protein secondary structure. The secondary structure of a protein is the arrangement in space of the atoms in the backbone portion of the protein. The two major types of protein secondary structure are the alpha helix and the beta pleated sheet.

Protein tertiary structure. The tertiary structure of a protein is the overall three-dimensional shape that results from the attractive forces among amino acid side chains (R groups).

Protein quaternary structure. The quaternary structure of a protein involves the associations among various polypeptide chains present in the protein.

Protein classifications. Proteins as classified as simple or conjugated on the basis of whether a non-amino-acid component is present. Proteins are classified as fibrous or globular on the basis of structural shape.

Protein hydrolysis. Protein hydrolysis is a chemical reaction in which peptide bonds within a protein are broken through reaction with water. Complete hydrolysis produces free amino acids.

Protein denaturation. Protein denaturation is the partial or complete disorganization of a protein's characteristic three-dimensional shape as a result of disruption of its secondary, tertiary, and quaternary structural interactions.

Enzyme. Enzymes are highly specialized protein molecules that act as biochemical catalysts. Enzymes have common names that provide information about their function rather than their structure. The suffix -ase is characteristic of most enzyme names.

Enzyme structure. Simple enzymes are composed only of protein (amino acids). Conjugated enzymes have a nonprotein portion (cofactor) in addition to a protein portion (apoenzyme). Cofactors may be small organic molecules (coenzymes) or inorganic ions.

Enzyme active site. An enzyme active site is the relatively small part of the enzyme that is actually involved in catalysis. It is where substrate binds to the enzyme.

Lock-and-key model of enzyme activity. The active site in an enzyme has a fixed, rigid geometrical conformation. Only substrates with a complementary geometry can be accommodated at the active site.

Induced-fit model of enzyme activity. The active site in an enzyme can undergo small changes in shape or geometry in order to accommodate a series of related substrates.

Enzyme activity. Enzyme activity is a measure of the rate at which an enzyme converts substrate to products. Four factors that affect enzyme activity are temperature, pH, substrate concentration, and enzyme concentration.