Serine solubility

Secondary Structure. The silkworm cocoon and spider dragline silks are characterized as an antiparaHel P-pleated sheet wherein the polymer chain axis is parallel to the fiber axis. Other silks are known to form a-hehcal (bees, wasps, ants) or cross- P-sheet (many insects) stmctures. The cross-P-sheets are characterized by a polymer chain axis perpendicular to the fiber axis and a higher serine content. Most silks assume a range of different secondary stmctures during processing from soluble protein in the glands to insoluble spun fibers. 

Factor II. Prothrombin is a vitamin K-dependent compound synthesized by the Hver. When prothrombin is activated it is cleaved at two sites, resulting in a two-chain molecule linked by a disulfide bond that has a molecular weight of 37,000 daltons. Thrombin is the serine protease that initiates the conversion of soluble fibrinogen into fibrin. 

FIGURE 9.31 The known proteoglycans include a variety of structures. The carbohydrate groups of proteoglycans are predominantly glycosaminoglycans O-linked to serine residues. Proteoglycans include both soluble proteins and integral transmembrane proteins. 

Plasmin, a serine protease (83 kDa), can degrade fibrin, and its degradation products (FDP) are soluble in the blood. Plasmin is formed from its proenzyme (zymogen, precursor), plasminogen (92 kDa), synthesized by the liver, and secreted into the blood circulation, where its concentration is 2 pM. Plasminogen is converted to plasmin by plasminogen activators (serine proteases). 

Scheme 18 Synthesis of water-soluble EG-grafted poly(L -serine)s...

These three compounds exert many similar effects in nucleotide metabolism of chicks and rats [167]. They cause an increase of the liver RNA content and of the nucleotide content of the acid-soluble fraction in chicks [168], as well as an increase in rate of turnover of these polynucleotide structures [169,170]. Further experiments in chicks indicate that orotic acid, vitamin B12 and methionine exert a certain action on the activity of liver deoxyribonuclease, but have no effect on ribonuclease. Their effect is believed to be on the biosynthetic process rather than on catabolism [171]. Both orotic acid and vitamin Bu increase the levels of dihydrofolate reductase (EC 1.5.1.4), formyltetrahydrofolate synthetase and serine hydroxymethyl transferase in the chicken liver when added in diet. It is believed that orotic acid may act directly on the enzymes involved in the synthesis and interconversion of one-carbon folic acid derivatives [172]. The protein incorporation of serine, but not of leucine or methionine, is increased in the presence of either orotic acid or vitamin B12 [173]. In addition, these two compounds also exert a similar effect on the increased formate incorporation into the RNA of liver cell fractions in chicks [174—176]. It is therefore postulated that there may be a common role of orotic acid and vitamin Bj2 at the level of the transcription process in m-RNA biosynthesis [174—176]. 

The most important reaction in blood clotting is the conversion, catalyzed by thrombin, of the soluble plasma protein fibrinogen (factor 1) into polymeric fibrin, which is deposited as a fibrous network in the primary thrombus. Thrombin (factor 11a) is a serine proteinase (see p. 176) that cleaves small peptides from fibrinogen. This exposes binding sites that spontaneously allow the fibrin molecules to aggregate into polymers. Subsequent covalent cross-linking of fibrin by a transglutaminase (factor Xlll) further stabilizes the thrombus. 

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