Food Biochemistry and Food Processing (2 edition)

BLBS102-c01 BLBS102-Simpson March 21, 2012 11:8 Trim: 276mm X 219mm Printer Name: Yet to Come

1 Introduction to Food Biochemistry 11

Table 1.5.Proteases in Animal Tissues and Their Degradation

Enzyme Reaction

Aspartic proteases

Pepsin A (pepsin, EC 3.4.23.1) Preferential cleavage, hydrophobic, preferably aromatic, residues in

P1 and P’1 positions

Gastricsin (pepsin C, EC 3.4.23.3) More restricted specificity than pepsin A; high preferential cleavage

at Tyr bond

Cathepsin D (EC 3.4.23.5) Specificity similar to, but narrower than that of pepsin A

Serine proteases

Trypsin (a- and b-trypsin, EC 3.4.21.4) Cleavage to the C-terminus of Arg and Lys

Chymotrypsin (Chymotrypsin A and B, EC 3.4.21.1) Preferential cleavage: Tyr-, Trp-, Phe-, Leu-

Chymotrysin C (EC 3.4.21.2) Preferential cleavage: Leu-, Tyr-, Phe-, Met-, Trp-, Gln-, Asn-

Pancreatic elastase (pancreato-peptidase E, pancreatic

elastase I, EC 3.4.21.36)

Hydrolysis of proteins, including elastin. Preferential cleavage: Ala

Plasmin (fibrinase, fibrinolysin, EC 3.4.21.7) Preferential cleavage: Lys>Arg; higher selectivity than trypsin

Enteropeptidase (enterokinase, EC 3.4.21.9) Activation of trypsinogen by selective cleavage of Lys6-Ile7 bond

Collagenase Hydrolysis of collagen into smaller molecules

Thio/cysteine proteases

Cathepsin B (cathepsin B1, EC 3.4.22.1) Broad speicificity, Arg–Arg bond preference in small peptides

Papain (EC 3.4.22.2) Broad specificity; preference for large, hydrophobic amino acid at

P2; does not accept Val at P1′

Fiacin (ficin, EC 3.4.23.3) Similar to that of papain

Bromelain (EC 3.4.22.4) Broad specificity similar to that of pepsin A

γ-Glutamyl hydrolase (EC 3.4.22.12 changed to 3.4.1.99) Hydrolysesγ-glutamyl bonds

Cathepsin H (EC 3.4.22.16) Protein hydorlysis; acts also as an aminopeptidase and

endopeptidase (notably cleaving Arg bond)

Calpain-1 (EC 3.4.22.17 changed to 3.4.22.50) Limited cleavage of tropinin I, tropomyosin, myofibril C-protein,

cytoskeletal proteins; activates phosphorylase, kinase, and

cyclic-nucleotide-dependent protein kinase

Metalloproteases

ProcollagenN-proteinase (EC 3.4.24.14) CleavesN-propeptide of pro-collagen chainα1(I) at Pro+Gln and

α1(II), andα2(I) at Ala+Gln

Source: Eskin 1990, Haard 1990, Lowrie 1992, Huff-Lonergan and Lonergan 1999, Gopakumar 2000, Jiang 2000, Simpson 2000, Greaser 2001,

IUBMB-NC website (www.iubmb.org).

food processing, e.g. deamidation, natural, enzymatic protein

modifications collectively known aspost-translational modifi-

cationsmay also occur upon their expression in cells. Some

examples are listed in Table 1.8.

Protein Structure

Protein folding largely occurs as a means to minimise the en-

ergy of the system where hydrophobic groups are maximally

shielded from aqueous environments and while the exposure

of hydrophilic groups to aqueous environments is maximised.

Protein structures follow a hierarchy: primary, secondary, ter-

tiary and quaternary structures. Primary structure refers to the

amino acid sequence; secondary structures are the structures

formed by amino acid sequences (e.g.α-helix,β-sheet, random

coil); tertiary structures are the 3D structures made up of sec-

ondary structures (the way that helices, sheets and random coils

pack together) and quaternary structure refers to the associa-

tion of tertiary structures (e.g. two subunits of an enzyme) in

oligomeric proteins. The overall shapes of proteins fall into two

general types: globular and fibrous. Enzymes, transport proteins

and receptor proteins are examples of globular proteins having

a compact, spherical shape. Hair keratin and muscle myosin are

examples of fibrous proteins having elongated structures that are

simple compared to globular proteins.

Oxidative Browning

Oxidative browning, also called enzymatic browning, involves

the actions of a group of enzymes generally referred to as

polyphenol oxidase (PPO) or phenolase. PPO is normally com-

partmentalised in tissue such that oxygen is unavailable. Injury

or cutting of plant material, especially apples, bananas, pears and

lettuce, results in decompartmentalisation, making O 2 available