Chymotrypsin, calcium binding

Figure 2.19 Organization of polypeptide chains into domains. Small protein molecules like the epidermal growth factor, EGF, comprise only one domain. Others, like the serine proteinase chymotrypsin, are arranged in two domains that are required to form a functional unit (see Chapter 11). Many of the proteins that are involved in blood coagulation and fibrinolysis, such as urokinase, factor IX, and plasminogen, have long polypeptide chains that comprise different combinations of domains homologous to EGF and serine proteinases and, in addition, calcium-binding domains and Kringle domains.

Fig. 3.13. Proportionality of the loss of accessible surface area to the molecular weight of proteins (from Janin, 1976). Different proteins insulin, rubredoxin, pancreatic trypsin inhibitor, HIPIP, calcium binding protein, ribonuclease S, lysozyme, staphylococcal nuclease, papain, chymotrypsin, concanavalin A, subtilisin, thermolysin, carboxypeptidase A.

In addition, peptides binding different minerals have been found in whey proteins, i.e., from (3-lg, a-la and LF. Since these proteins are not phosphorylated, the minerals seem to bind through other binding sites than caseins. Seventeen (17) different peptides have been identified by hydrolysis of (3-lg with thermolysin using two different concentrations of calcium. Also, peptides from a-la and LF using trypsin, chymotrypsin or pepsin have been reported. Studies with 3-lg and a-la peptides have shown a higher affinity for iron than the native proteins (Vegarud et al., 2000). 

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