All Peptides and Polypeptides Are Polymers of Amino Acids. There Are 20 Amino Acids That

Proteins

All peptides and polypeptides are polymers of amino acids. There are 20 amino acids that are relevant to the chemical make-up of mammalian proteins.

Amino Acids

Consist of a carboxylic acid (-COOH) and an amino (-NH2) functional group attached to an α-carbon. Distinct R-groups that distinguish one amino acid from another are also attached. The final bond on the α-carbon is a hydrogen.

Amino Acid Classifications

Each of the 20 amino acids can be distinguished by the R-groups. Two main classifications of R-groups are hydrophobic and hydrophilic.

The hydrophobic amino acids tend to repel water and therefore usually reside in the interior of proteins. The hydrophilic amino acids tend to interact with water and are usually found on the exterior surfaces of proteins or in the active sites of enzymes.

Acid-Base Properties of Amino Acids:

R-COOH <——> R-COO– + H+

R-NH3+ <——> R-NH2+ H+

The –COOH and the –NH2 groups and the various R-groups are capable of ionizing.

Amino acids are the best known examples of Zwitterions – neutral molecules with a positive and negative electrical charge at different locations within that molecule.

The Peptide Bond

Peptide bond formation is a condensation reaction (dehydration synthesis) leading to the polymerization of amino acids into peptides and proteins. Peptides are smallconsisting of a few amino acids. A number of hormones and neurotransmitters are peptides. A dipeptide contains a single peptide bond formed by the condensation of a carboxyl group of one amino acid with the amino group of the other amino acid.

Primary Structure of Proteins

Refers to the linear number and order of the amino acids present.

Secondary Structure in Proteins

Within a single protein, different regions of the polypeptide chain assume different conformations determined by the primary sequence of amino acids.

α-Helix: formation is stabilized by H-bonds between amino nitrogens and carbonyl carbons of the peptide bonds every 4 amino acids apart. Helical coiling of the peptide backbone results

- Keratin (hair), Myosin (muscles), Fibrin (blood clots) are 2 or more helices coiled together

Β-Sheets: composed of 2 or more different regions of stretches of at least 5-10 amino acids. Β-sheets are said to be pleated.

- H-bonding occurs between neighboring polypeptide chains rather than within one as in the α-helix

- Less common than the α-helix.

-Found extensively only in the protein of silk

Tertiary Structure in Proteins

Quaternary Structure in Proteins

Many proteins contain 2 or more different polypeptide chains that are held together.

Ex: Hemoglobin, the oxygen carrying protein of the blood, contains 2 α and 2 β subunits.