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Chemical Nature of the Amino Acids

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All peptides and polypeptides are polymers of α-amino acids. There are 20 α-amino acids that are relevant to the make-up of mammalian proteins (see below). Several other amino acids are found in the body free or in combined states (i.e. not associated with peptides or proteins). These non-protein associated amino acids perform specialized functions. Several of the amino acids found in proteins also serve functions distinct from the formation of peptides and proteins, e.g., tyrosine in the formation of thyroid hormones or glutamate acting as a neurotransmitter.

Table of α-Amino Acids Found in Proteins

Amino Acid Symbol Structure* pK1 (COOH) pK2 (NH2) pK R Group
Amino Acids with Aliphatic R-Groups
Glycine Gly – G Structure of glycine 2.4 9.8
Alanine Ala – A Structure of alanine 2.4 9.9
Valine Val – V Structure of valine 2.2 9.7
Leucine Leu – L Structure of leucine 2.3 9.7
Isoleucine Ile – I Structure of isoleucine 2.3 9.8
Non-Aromatic Amino Acids with Hydroxyl R-Groups
Serine Ser – S Structure of serine 2.2 9.2 ≈13
Threonine Thr – T Structure of threonine 2.1 9.1 ≈13
Amino Acids with Sulfur-Containing R-Groups
Cysteine Cys – C Structure of cysteine 1.9 10.8 8.3
Methionine Met – M Structure of methionine 2.1 9.3
Acidic Amino Acids and their Amides
Aspartic Acid Asp – D Structure of aspartic acid 2.0 9.9 3.9
Asparagine Asn – N Structure of asparagine 2.1 8.8
Glutamic Acid Glu – E Structure of glutamic acid 2.1 9.5 4.1
Glutamine Gln – Q Structure of glutamine 2.2 9.1
Basic Amino Acids
Arginine Arg – R Structure of arginine 1.8 9.0 12.5
Lysine Lys – K Structure of lysine 2.2 9.2 10.8
Histidine His – H Structure of histidine 1.8 9.2 6.0
Amino Acids with Aromatic Rings
Phenylalanine Phe – F Structure of phenylalanine 2.2 9.2
Tyrosine Tyr – Y Structure of tyrosine 2.2 9.1 10.1
Tryptophan Trp – W Structure of tryptophan 2.4 9.4
Imino Acids
Proline Pro – P Structure of proline 2.0 10.6

The Peptide Bond

Peptide bond formation is a condensation reaction leading to the polymerization of amino acids into peptides and proteins. Peptides are small consisting of few amino acids. A number of hormones and neurotransmitters are peptides. Additionally, several antibiotics and antitumor agents are peptides. Proteins are polypeptides of greatly divergent length. The simplest peptide, a dipeptide, contains a single peptide bond formed by the condensation of the carboxyl group of one amino acid with the amino group of the second with the concomitant elimination of water. The presence of the carbonyl group in a peptide bond allows electron resonance stabilization to occur such that the peptide bond exhibits rigidity not unlike the typical –C=C– double bond. The peptide bond is, therefore, said to have partial double-bond character.

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