Proteins and Amino Acids

A protein is a chain of amino acids. At physiological pH, amino acids exist predominantly in a zwitterionic form, carrying both, a positive and a negative charge. Each amino acid contains amine and carboxyl groups, along with a different side group, denoted as R, attached to a core carbon.

Amino acids are linked through covalent peptide bonds, and the repeating series of atoms along the core is called the polypeptide backbone.

The sequence begins at the N-terminus, with a free amino group, NH₃⁺, and ends with the C-terminus, a free carboxyl group, COO^-.

Each protein is unique, depending on the number of amino acids and their side chains. For example, thyroid-releasing hormone has 3 amino acids, whereas the elastic muscle protein titin has over 34,000.

There are 20 standard amino acids, each with a different side chain, and they fall into one of four groups.

Negative polar amino acids, such as aspartic and glutamic acid, have side chains with a carboxyl group. Because of this, they carry a negative charge at physiological pH.

Positive polar amino acids carry a positive charge at physiological pH and have nitrogen-containing side chains.

Several amino acids are polar but uncharged. They are