Food Biochemistry and Food Processing

8 Enzyme Engineering and Technology 177

R 1 R 2 R 1 HH

⏐⏐
H 20 ⏐⏐⏐

H 2 N⎯C⏐⏐⎯COOH H 2 N⎯C⎯COOH ⎯→H 2 N⎯C⏐⎯Ck⎯N⎯C⏐⎯COOH

HH HOR 2

Peptitde Bond

There are 20 common amino acids in proteins that
are specified by the genetic code; in rare cases, oth-
ers occur as the products of enzymatic modifications
after translation. A common feature of all 20 amino
acids is a central carbon atom (C
) to which a hydro-
gen atom, an amino group (—NH 2 ) and a carboxyl
group (—COOH) are attached. Free amino acids are
usually zwitterionic at neutral pH, with the carboxyl
group deprotonated and the amino group protonat-
ed. The structures of the most common amino acids
found in proteins are shown in Table 8.1. Amino
acids can be divided into four different classes
depending on the structure of their side chains,

which are called R groups: nonpolar, polar un-

charged, negatively charged(at neutral pH), and

positively charged (at neutral pH) (Richardson

1981). The properties of the amino acid side chains

determine the properties of the proteins they consti-

tute. The formation of a succession of peptide bonds

generates the main chainor backbone.

The primary structure of a protein places several

constraints on how it can fold to produce its three-

dimensional structure (Cantor 1980, Fersht 1999).

The backbone of a protein consists of a carbon atom

C
to which the side chain is attached, a NH group

bound to C
, and a carbonyl group CuO, where the

carbon atom C is attached to C
(Fig. 8.1). The pep-

tide bond is planar, since it has a partial (
40%)

double bond character with  electrons shared

between the C—O and C—N bonds (Fig. 8.1). The

peptide bond has a trans-conformation, that is, the

Table 8.1.Names, Symbol (One Letter and Three Letter Codes), and Chemical Structures of
the Twenty Amino Acids Found in Proteins

Nonpolar Amino Acids

Alanine Ala (A)

Valine Val (V)

Leucine Leu (L)

Isoleucine Ile (I)

Methionine Met (M)

Tryptophan Trp (W)

Phenylalanine Phe (F)

Proline Pro (P)

Polar Uncharged Amino Acids

Glycine Gly (G)

(Continues)