Peptide, amino acid sequence cross-linked

The most abundant milk protein is casein, of which there are several different kinds, usually designated a-, (1-, and K-casein. The different caseins relate to small differences in their amino acid sequences. Casein micelles in milk have diameters less than 300 nm. Disruption of the casein micelles occurs during the preparation of cheese. Lactic acid increases the acidity of the milk until the micelles crosslink and a curd develops. The liquid portion, known as whey, containing water, lactose and some protein, is removed. Addition of the enzyme rennet (chymosin) speeds up the process by hydrolysing a specific peptide bond in K-casein. This opens up the casein and encourages further cross-linking. 

Oxytocin is a 9-amino-acid peptide with an intrapeptide disulfide cross-link (Figure 37-5). Its amino acid sequence differs from that of vasopressin at positions 3 and 8. 

The primary level of structure in a protein is the linear sequence of amino acids as joined together by peptide bonds. This sequence is determined by the sequence of nucleotide bases in the gene encoding the protein (see Topic HI). Also included under primary structure is the location of any other covalent bonds. These are primarily disulfide bonds between cysteine residues that are adjacent in space but not in the linear amino acid sequence. These covalent cross-links between separate polypeptide chains or between different parts of the same chain are formed by the oxidation of the SH groups on cysteine residues that are juxtaposed in space (Fig. 4). The resulting disulfide is called a cystine residue. Disulfide bonds are often present in extracellular proteins, but are rarely found in intracellular proteins. Some proteins, such as collagen, have covalent cross-links formed between the side-chains of Lys residues (see Topic B5). 

Specific amino acid sequences promote cell adhesion. The most commonly used sequence is the RGDs from adhesion proteins. Photo-cross-linking RGD peptides to chitosan improved the adhesion of human endothelial cells, compared to unmodified chitosan scaffolds [109]. In another approach, the -COOH group of amino acids such as lysine, arginine, aspartate, and phenylalanine reacts to the -NH group of chitosan. These functionalized chitosan polymers were entrapped onto the surface of PLA to improve cellular responses [110]. 

Progress in the Insulin field has been rapid in these two years, particularly in the axea, of the blos thesis of the hormone. It is now well established that Insulin, which has two peptide chains (A, 21 amino acids and B, 30 amino acids) cross-linked by two disulfide bridges, is synthesized as a single peptide chain, prolnsulin. in which the A and B chains of inmUn are connected by a "connecting peptide" (C-peptide) chain of 33 (porcine) or 30 (bovine) amino acids. Work on prolnsulin has be reviewed, 1, 10 and the amino acid sequences of bovine and porcine proinsulins have been published. Hie amino acid composition of cod proinsulin has a o appeared Two different proinsulins have been demonstrated in the rat, 8 aod proinsulin has been Isolated from human islet cell tumor tissue cultures. The structures of porcine and bovine prolnsulins are as follows ... 

The primary structure of a protein is the sequence of amino acids in its chain(s). Primary structure, which determines the other three kinds of structure, is maintained by the covalent peptide bonds between individual amino acids. The primary structure of egg-white lysozyme—a protein that helps fight infection—is shown in Figure 21.10 . The figure illustrates the amino acid sequence, the N-terminal and C-terminal ends, and the presence of disulfide linkages, covalent cross-links between cysteine amino acids in the polymer. We discuss disulfide linkages in more detail later in the section on tertiary structure. Researchers determined the first amino acid sequences for proteins in the 1950s. Today, the amino acid sequences for thousands of proteins are known. 

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