Food Biochemistry and Food Processing (2 edition)

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15 Biochemistry of Raw Meat and Poultry 289

Table 15.2.Type, Localization and Main Role of Major Muscle Proteins

Cellular

Localization Main Proteins Main Role in Muscle

Myofibrillar Myosin and actin Cytoskeletal proteins providing support to the myofibril and

responsible for contraction–relaxation of the muscle.

Tropomyosin Regulatory protein associated to troponins than cause its movement

toward the F-actin helix to permit contraction.

Troponins T, C, I Regulatory proteins. Tn-T binds to tropomyosin, Tn-C binds Ca^2 +

and initiates the contractile process and Tn-I inhibits the

actin–myosin interaction in conjunction with tropomyosin and

Tn-T and Tn-C.

Titin and nebulin Large proteins located between Z-lines and thin filaments. They

provide myofibrils with resistance and elasticity.

α-,β-,γ-, and eu-actinin Proteins regulating the physical state of actin.α-actinin acts as a

cementing substance in the Z-line.γ-actinin inhibits the

polymerization of actin at the nucleation step.β-actinin is located

at the free end of actin filaments, preventing them to bind each

other. Eu-actinin interacts with both actin andα-actinin.

Filamin, synemin, vinculin, zeugmatin,

Z-nin, C, H, X, F, I proteins

Proteins located in the Z-line that contribute to its high density.

Desmin Protein that links adjacent myofibrils through Z-lines.

Myomesin, creatine kinase and

M-protein

Proteins located in the center of the sarcomere forming part of the

M-line.

Sarcoplasm Mitochondrial enzymes Enzymes involved in the respiratory chain.

Lysosomal enzymes Digestive hydrolases very active at acid pH (cathepsins, lipase,

phospholipase, peptidases, glucohydrolases, etc.).

Other cytosolic enzymes Neutral proteases, lipases, glucohydrolases, ATPases, etc.

Myoglobin Natural pigment of meat.

Hemoglobin Protein present from remaining blood within the muscle.

Connective

tissue

Collagen Protein giving support, strength, and shape to the fibers,

Elastin Protein that gives elasticity to tissues like capillaries, nerves,

tendons, etc.

Source: Bandman 1987, Pearson and Young 1989.

meats with high content of fat have lower content of moisture

and proteins. A brief description of proteins and lipids as major

components of meat is given later.

Muscle Proteins

Proteins constitute the major compounds in the muscle, approx-

imately 15–22%, and have important roles for the structure, nor-

mal function, and integrity of the muscle. Proteins experience

important changes during the conversion of muscle to meat that

mainly affect tenderness; and additional changes occur during

further processing, through the generation of peptides and free

amino acids as a result of the proteolytic enzymatic chain. There

are three main groups of proteins in the muscle (see Table 15.2):

myofibrillar proteins, sarcoplasmic proteins, and connective tis-

sue proteins.

Myofibrillar Proteins

These proteins are responsible for the basic myofibrillar struc-

ture, and thus they contribute to the continuity and strength of

the muscle fibers. They are soluble in high ionic strength buffers,

and the most important of them are listed in Table 15.2. Myosin

and actin are by far the most abundant and form part of the struc-

tural backbone of the myofibril. Tropomyosin and troponins C,

T, and I are considered as regulatory proteins because they play

an important role in muscle contraction and relaxation (Pearson

1987). There are many proteins in the Z-line region (although

in a low percentage) that serve as bridges between the thin fil-

aments of adjacent sarcomeres. Titin and nebulin are two very

large proteins, present in a significant proportion, that are lo-

cated in the void space between the filaments and the Z-line

and contribute to the integrity of the muscle cells (Robson et al.

1997). Desmin is located on the external area of the Z-line and

connects adjacent myofibrils at the level of the Z-line.

Sarcoplasmic Proteins

These are water-soluble proteins, comprising about 30–35%

of the total protein in muscle. Sarcoplasmic proteins contain

a high diversity of proteins (summarized in Table 15.2), mainly