Food Biochemistry and Food Processing (2 edition)

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16 Biochemistry of Processing Meat and Poultry 305

Proteasome Complex

The proteasome is a multicatalytic complex with different func-

tions in living muscle, even though its role in postmortem mus-

cle is still not well understood. The 20S proteasome has a large

molecular mass, 700 kDa, and a cylinder structure with sev-

eral subunits. Its activity is optimal at pH above 7.0, but it

rapidly decreases when pH decreases, especially below 5.5. It

exhibits three major activities: (1) chymotrypsin-like activity,

(2) trypsin-like activity, and (3) peptidyl-glutamyl hydrolyzing

activity (Coux et al. 1996). This multiple activity behavior is the

reason why there was originally some confusion among labora-

tories over its identification. The 20S proteasome concentration

is higher in oxidative muscles than in glycolytic ones (Dahlmann

et al. 2001). This enzyme has shown degradation of some my-

ofibrillar proteins such as troponin C and myosin light chain and

could be involved in postmortem changes in slow twitch oxida-

tive muscles or in high-pH meat, where an enlargement of the

Z-line with more or less density loss is observed (Sentandreu

et al. 2002).

A new family of peptidases, named as caspases or apoptosis-

generating peptidases, are cysteine peptidases that have been

recently proposed to be involved in cell death and thus immediate

postmortem changes in proteins having some impact on the

phases of rigor and meat aging. Three main pathways of cellular

death development have been proposed (Herrera-M ́endez et al.

2006). These peptidases are active at neutral pH, and one of the

limitations to its activity in postmortem meat is the acid pH in

the postmortem muscle.

Exoproteases: Peptidases

There are several peptidases in the muscle with the ability to

release small peptides of importance for taste. Tripeptidylpep-

tidases (TPPs) are enzymes capable of hydrolyzing different

tripeptides from the amino termini of peptides, while dipep-

tidylpeptidases (DPPs) are able to hydrolyze different dipeptide

sequences. There are two TPPs and four DPPs, and their molec-

ular masses are relatively high, between 100 and 200 kDa, or

even as high as 1000 kDa for TPP II, and have different substrate

specificities. TPP I is located in the lysosomes, has an optimal

acid pH (4.0), and is able to hydrolyze tripeptides Gly-Pro-X,

where X is an amino acid, preferentially of hydrophobic na-

ture. TPP II has optimal neutral pH (6.5–7.5) and wide substrate

specificity, except when Pro is present on one of both sides of the

hydrolyzed bond. DPPs I and II are located in the lysosomes and

have optimal acid pH (5.5). DPP I has a special preference for

hydrolyzing the dipeptides Ala-Arg and Gly-Arg, while DPP

II prefers a terminal Gly-Pro sequence. DPP III is located in

the cytosol and has special preference for terminal Arg-Arg and

Ala-Arg sequences. DPP IV is linked to the plasma membrane

and prefers a terminal Gly-Pro sequence. Both DPP III and IV

have an optimal pH around 7.5–8.0. All these peptidases have

been purified and fully characterized in porcine skeletal muscle

(Toldra 2002). ́

Exoproteases: Aminopeptidases and Carboxypeptidases

There are five aminopeptidases, known as leucyl, arginyl, alanyl,

pyroglutamyl, and methionyl aminopeptidases, based on their

respective preference or requirement for a specificN-terminal

amino acid. They are able, however, to hydrolyze other amino

acids, although at a lower rate (Toldr ́a 1998). Aminopeptidases

are metalloproteases with a very high molecular mass and com-

plex structures. All of them are active at neutral or basic pH.

Alanyl aminopeptidase, also known as the major aminopepti-

dase because it exhibits very high activity, is characterized by its

preferential hydrolysis of alanine, but it is also able to act against

a wide spectrum of amino acids such as aromatic, aliphatic, and

basic aminoacyl bonds. Methionyl aminopeptidase has prefer-

ence for methionine, alanine, lysine, and leucine, but also has a

wide spectrum of activity. This enzyme is activated by calcium

ions. Arginyl aminopeptidase, also known as aminopeptidase B,

hydrolyzes basic amino acids such as arginine and lysine (Toldr ́a

and Flores 1998).

Carboxypeptidases are located in the lysosomes and have op-

timal acid pH. They are able to release free amino acids from

the carboxy termini of peptides and proteins. Carboxypeptidase

A has preference for hydrophobic amino acids, whereas car-

boxypeptidase B has a wide spectrum of activity (McDonald

and Barrett 1986).

Lipolytic Enzymes

Lipolytic enzymes are characterized by their ability to degrade

lipids, and they receive different names depending on their mode

of action (see Fig. 16.2). They are known as lipases when they

are able to release long-chain fatty acids from triacylglycerols,

while they are know as esterases when they act on short-chain

fatty acids. Lipases and esterases are located either in the skeletal

muscle or in the adipose tissue. Phospholipases, mainly found

in the skeletal muscle, hydrolyze fatty acids at positions 1 or 2

in phospholipids.

O

Lipase

O

C

HCOH R^11 Phospholipase A^1 O

2 CO

OCH

R 2

O

C

C HCO R 1

H 2 CO R

3

H 2 CO

OCH

R 2

C

O

C

O

3

H 2 CO

P

O

O

P

Phospholipase A (^2) O-
OCH 2 CH 2 (CH) 3
Figure 16.2.Mode of action of muscle lipases and phospholipases.