Protein Processing in Food and Bioproduct Manufacturing and Techniques for Analysis - Food and Industrial Bioproducts and Bioprocessing - page 84

3.2 GENERAL PROPERTIES OF PROTEINS

Proteins are comprised of amino acids linked by peptide bonds (Figure 3.1). A list of the 20
amino acids most commonly found in nature is presented in Table 3.1. Other rare amino
acids found in nature include ornithine, citrulline and cycloleucine. Amino acid exists in the
D or L conformation depending on their rotatory optical activity (Figure 3.2). Most amino
acids found in nature exist in the L conformation.
Proteins have complex molecular structures. The linear sequence of the amino acids
comprising a protein is classified as its primary structure. In different proteins, these linear
sequences assume conserved structures along the axis of the polypeptide in the form of
alpha-helixes, 3 10 -helix, beta sheets, or random coils, turns which are described as the
secondary structure of the protein. These secondary structures are stabilized primarily by
hydrogen bonds. For thermodynamic stability, proteins rearrange themselves into tertiary
structures comprising several secondary structures stabilized by van der Waal's, electro-
static, or hydrophobic interactions, hydrogen bonding, as well as disulfide cross-links. Some
proteins have a fourth structural level called the quaternary structure in which two or more

H O R H O R H

| || | | || | | |

N C CH N C CH N

CH CH CH

N C N C

| | || | | || |

R H O R H O R

Figure 3.1 Amino acids linked by peptide bonds.

Table 3.1 The 20 amino acids most commonly found in nature and their characteristics.

Amino Acid Abbrev. Letter Type MW

Alanine ala A Aliphatic 89.1

Arginine arg R Basic 174.2

Asparagine asn N Amidic 132.1

Aspartic acid asp D Acidic 133.1

Cysteine cys C Sulfur containing 121.1

Glutamine gln Q Amidic 146.1

Glutamic acid glu E Acidic 147.1

Glycine gly G Aliphatic 75.1

Histidine his H Basic 155.2

Isoleucine ile I Aliphatic 131.2

Leucine leu L Aliphatic 131.2

Lysine lys K Basic 146.2

Methionine met M Sulfur containing 149.2

Phenylalanine phe F Aromatic 165.2

Proline pro P Aliphatic 115.1

Serine ser S Hydroxylic 105.1

Threonine thr T Hydroxylic 119.1

Tryptophan trp W Aromatic 204.2

Tyrosine tyr Y Aromatic 181.2

Valine val V Aliphatic 117.1