amino acid

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

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PRIMARY STRUCTURE OF PROTEINS
Amino acids are link together in a chain by condensation of the alpha-carboxyl group of one amino acid with the alpha-amino group of another. The linkage is then an amide bond called peptide bond.

The linked amino acid moieties are called residues. When glycine is found in a polypeptide chain it is called a glycyl residue.
The free amino group and the free carboxylic group at the opposite ends of the peptide chain are called the N-terminus and the C-terminus, respectively. At neutral pH, each terminus carries an ionic charge. Amino acid residues are numbered from the N-terminus to the C-terminus and are written from left to right.

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