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5. Proteins in structures Page 21
diagram of filament
Figure 8
The structure of a thin filament in the relaxed state.
5.3 Muscle - continued
Many small globular actin molecules collect together to make an actin filament. The actin filament looks like two strings of pearls twisted together (Figure 8). The thin filaments also contain the coiled protein tropomyosin, which winds through the grooves in the actin filament. Globular troponin molecules bind at sites along the tropomyosin.

Binding of the myosin heads to points along the thin filament happens when chemical messengers, the calcium ions, cause conformational changes in the troponin molecules. These cause conformational changes in the tropomyosin and then in actin, exposing regions along the actin molecule where the myosin heads can stick.

Figure 9
Small conformational changes make muscles contract.
Myosin molecules have flexible regions near to their heads. The breakdown of ATP molecules provides energy for the myosin heads to flip about these regions and drag the thin filaments past the thick filaments (Figure 9).

Myofibrillar proteins (actin and myosin), connective tissue and fat are the main components in meat. When an animal is slaughtered the supply of ATP runs out, causing the actin and myosin to gradually lock together. The actomyosin molecules formed are inflexible, making the muscle rigid and producing rigor mortis.

In some countries you can buy meat from recently slaughtered animals to cook and eat before the toughening process of rigor has started. Once rigor has developed the meat may be made more tender by conditioning. Conditioning involves hanging the animal carcass for several days to resolve the rigor. The changes that occur are complex, but food technologists believe that the meat becomes more tender as the structural proteins in the myofibrils and connective tissue are subtly altered.

Ca2+ ions are released by muscle tissue when it receives a nerve impulse. These ions are the chemical ’messengers’ that bind to the troponin protein.

1. What groups in the troponin molecules are able to hold on to Ca2+ ions?

2. Myosin molecules need energy to enable them to flip. Myosin molecules are catalysts . Which reaction do you think they catalyse?

Unilever Education Advanced Series: Proteins
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