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Amino Acid Properties and Structure

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Amino acids are building blocks of proteins connected by a specific type of covalent linkage. Amino acids are vital to life because the proteins consisting of them are involved in almost all cellular functions. Some proteins act as enzymes, some as antibodies, and others provide structural support. In addition to serving as building blocks for peptides and proteins, amino acids (either directly or in their modified form) are involved in many metabolic functions. They include neurotransmitter functions as well as precursors for the synthesis of non-protein nitrogen-containing metabolites. Although hundreds of amino acids have been found in nature, proteins are made up of 20 amino acids.

The basic structure of an amino acid consists of an amino group (-NH2), a carboxylic acid group (-COOH), a hydrogen atom, and a unique side chain termed "R-group". Each molecule contains a central carbon atom, known as α-carbon, to which both the amino and carboxylic acid groups are attached. The remaining two bonds of the α-carbon atom are usually satisfied by a hydrogen atom and an R group. Thus, the α-carbon atom is attached to four substituents. Amino acids differ from each other in the specific chemical properties and structure of the R groups.

The amino acid structure diagram.Figure 1. The amino acid structure diagram.

Name/ CodeM.W.pIpKa, NH2pKa, COOHpK (R)SolubilityFormula/ Structure3D Structure
Alanine/ Ala (A)89.106.009.872.35-15.8C3H7NO2

Ala (A)

Ala (A)

Arginine/ Arg (R)174.2010.769.092.1813.271.8C6H14N4O2

Arg (R)

Arg (R)

Asparagine/ Asn (N)132.125.418.82.02-2.4C4H8N2O3

Asn (N)

Asn (N)

Aspartic Acid/ Asp (D)133.112.779.61.883.650.42C4H7NO4

Asp (D)

Asp (D)

Cysteine/ Cys (C)121.165.0710.781.718.33freelyC3H7NO2S

Ala (A)

Glutamic Acid/ Glu (E)147.133.229.672.194.250.72C5H9NO4

Glu (E)

Glutamine/ Gln (Q)146.155.659.132.17-2.6C5H10N2O3

Ala (A)

Gln (Q)

Glycine/ Gly (G)75.075.979.62.34-22.5C2H5NO2

Gly (G)

Gly (G)

Histidine/ His (H)155.167.598.971.7864.19C6H9N3O2

His (H)

His (H)

Isoleucine/ Ile (I)131.186.029.762.32-3.36C6H13NO2

Ile (I)

Ile (I)

Leucine/ Leu (L)131.185.989.62.36-2.37C6H13NO2

Leu (L)

Leu (L)

Lysine/ Lys (K)146.199.7410.288.92.2-C6H14N2O2

Lys (K)

Lys (K)

Methionine/ Met (M)149.215.749.212.28-5.14C5H11NO2S

Met (M)

Met (M)

Phenylalanine/ Phe (F)165.195.489.242.58-2.7C9H11NO2

Phe (F)

Phe (F)

Proline/ Pro (P)115.136.3010.61.99-1.54C5H9NO2

Pro (P)

Pro (P)

Serine/ Ser (S)105.095.689.152.21-36.2C3H7NO3

Ser (S)

106

Threonine/ Thr (T)119.125.609.122.15-freelyC4H9NO3

Ala (A)

Thr (T)

Tryptophan/ Trp (W)204.235.899.392.38-1.06C11H12N2O2

Trp (W)

Trp (W)

Tyrosine/ Tyr (Y)181.195.669.112.210.10.038C9H11NO3

Tyr (Y)

Tyr (Y)

Valine/ Val (V)117.155.969.722.29-5.6C5H11NO2

Val (V)

Val (V)

Table 1. Amino acid properties and structure.

Amino acid classification

Based on the properties of the R group, the 20 common amino acids are classified as follows:

Amino acid classification.Figure 2. Amino acid classification.

How are amino acids joined together?

The amino acid can be connected by a condensation reaction in which the nucleophilic addition-elimination reaction between the amino group of one amino acid and the carboxyl group of another amino acid forms a covalent bond (peptide bond), and releases one molecule of water as a by-product. And the peptide bond is essentially an amide bond.

Amino acids linked by a series of peptide bonds constitute a peptide chain, each amino acid of which is referred to as an amino acid residue. Polymers composed of less than 50 amino acid residues are termed oligopeptides, while larger polymers (more than 50 amino acid residues) are known as polypeptides. Thus, a protein molecule is a polypeptide chain composed of many amino acid residues, with each residue linked to the next residue by a peptide bond.

Amino acid classification.Figure 3. Peptide bond formation.

Creative Biostructure is specialized in the field of structural biology, and we provide contract services for the structural analysis of proteins of your interest.

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