Food Biochemistry and Food Processing

316 Part III: Muscle Foods

DESCRIPTION OF THE MUSCLE
ENZYMES

There are a wide variety of enzymes in the muscle.
Most of them have an important role in the in vivo
muscle functions, but they also serve an important
role in biochemical changes such as the proteolysis
and lipolysis that occur in postmortem meat, and
during further processing of meat. Some enzymes
are located in the lysosomes, while others are free in
the cytosol or linked to membranes. The muscle en-
zymes with most important roles during meat pro-
cessing are grouped by families and described below.

MUSCLEPROTEASES

Proteases are characterized by their ability to de-
grade proteins, and they receive different names de-
pending on respective mode of action (see Fig.
14.1). They are endoproteases or proteinases when
they are able to hydrolyze internal peptide bonds,
but they are exopeptidases when they hydrolyze
external peptide bonds, either at the amino termini
or the carboxy termini.

Neutral Proteinases: Calpains

Calpains are cysteine endopeptidases consisting of

heterodimers of 110 kDa, composed of an 80 kDa

catalytic subunit and a 30 kDa subunit of unknown

function. They are located in the cytosol, around the

Z-line area. Calpains have received different names

in the scientific literature, such as calcium-activated

neutral proteinase, calcium-dependent protease, and

calcium-activated factor. Calpain I is also called 
-

calpain because it needs micromolar amounts (50–

70 
M) of Ca^2 
for activation. Similarly, calpain II

is called m-calpain because it requires millimolar

amounts (1–5 mM) of Ca^2 
. Both calpains show

maximal activity around pH 7.5. Calpain activity

decreases very quickly when pH decreases to 6.0, or

even reaches ineffective activity at pH 5.5 (Ether-

ington 1984). Calpains have shown good ability to

degrade important myofibrillar proteins, such as titin,

nebulin, troponins T and I, tropomyosin, C-protein,

filamin, desmin, and vinculin, which are responsible

for the fiber structure. On the other hand, they are

not active against myosin, actin, -actinin and tro-

ponin C (Goll et al. 1983, Koohmaraie 1994).

Figure 14.1.Mode of action of the different types of muscle proteases.