The Structure of Proteins
Proteins are polymers of amino acids covalently linked through peptide bonds into a chain.
Proteins are polymers of amino acids joined together by peptide bonds. There are 20 different amino acids that make up essentially all proteins on earth. Each of these amino acids has a fundamental design composed of a central carbon (also called the alpha carbon) bonded to:
- a hydrogen
- a carboxyl group
- an amino group
- a unique side chain or R-group
Peptides and Proteins
Amino acids are covalently bonded together in chains by peptide bonds. If the chain length is short (say less than 30 amino acids) it is called a peptide; longer chains are called polypeptides or proteins. Peptide bonds are formed between the carboxyl group of one amino acid and the amino group of the next amino acid. Peptide bond formation occurs in a condensation reaction involving loss of a molecule of water.
Levels of Protein Structure
Structural features of proteins are usually described at four levels of complexity:
- Primary structure: the linear arrangement of amino acids in a protein and the location of covalent linkages such as disulfide bonds between amino acids.
- Secondary structure: areas of folding or coiling within a protein; examples include alpha helices and pleated sheets, which are stabilized by hydrogen bonding.
- Tertiary structure: the final three-dimensional structure of a protein, which results from a large number of non-covalent interactions between amino acids.
- Quaternary structure: non-covalent interactions that bind multiple polypeptides into a single, larger protein. Hemoglobin has quaternary structure due to association of two alpha globulin and two beta globulin poly-proteins.