Mucins amino acids

Mucins Have a High Content of O-Linked Oligosaccharides Exhibit Repeating Amino Acid Sequences... 

Saliva High Amino acids and mucin Minimal Hydrophilic and hydrophobic components... 

Mammals have two isozymes of prostaglandin H2 synthase, COX-1 and COX-2. These have different functions but closely similar amino acid sequences (60% to 65% sequence identity) and similar reaction mechanisms at both of their catalytic centers. COX-1 is responsible for the synthesis of the prostaglandins that regulate the secretion of gastric mucin, and COX-2 for the prostaglandins that mediate inflammation, pain, and fever. Aspirin inhibits both isozymes about equally, so a dose sufficient to reduce inflammation also risks stomach irritation. Much research is aimed at developing new NSAIDs that inhibit COX-2 specifically, and several such drugs have become available. 

On the basis of the close correlation between the number of hydroxy-amino acids in bovine submaxillary mucin and the number of hexosamine and sialic acid residues, Hashimoto and Pigman (34) proposed that an O-glycosidic linkage was the only one that could be present in large amounts. Anderson et al. (38) reported a disappearance of much of the serine after incubation of the proteoglycan of chondroitin sulfate with alkali and suggested that this loss resulted from a -elimination reaction. 

Core structures of mucin-type O-linked glycans. These represent a series of characteristic structures attached to a serine (S) or threonine (T) amino acid residue of a protein. There are eight core structures, each of which may have further monosaccharide additions. Tn is a precursor of theT (Thomsen-Freidenreich) antigen which is commonly used as a marker for tumor cells. 

Mucin polypeptides comprise thousands of amino acid residues organized into different protein domains with specific structural and/or functional properties (Fig. 2.1) (Perez-Vilar and Hill 1999 Dekker et al. 2002). 

D-domain, which contains up to 30 cysteines and up to 400 amino acid residues, shows significant sequence identity with the other D-domains, especially the cysteine, glycine, and proline residues. Further analysis of the D-domain sequences has defined a potential domain, known as the trypsin inhibitor-like cysteine-rich domain (TIL) within each D-domain sequence (Lang et al. 2004). The NH2-terminal D-domains are involved in formation of mucin disulfide-linked oligomers/multimers (Perez-Vilar and Hill 1999 Perez-Vilar and Mabolo 2007 see Section 2.3.2). 

Fig. 2.2 Different levels of structural organization in gel-forming mucins. From the biochemical point of view (a), mucin polypeptides have very complex multi-domain structures and glycosylation patterns and thousands of amino acids per monomer. Moreover, the monomers are assembled into disulfide-linked oligomers/multimers that have contour sizes of several microns.

Sialic acids are a family of nine-carbon acidic monosaccharides based on 3-deoxy nonulosonic acid (Fig. 17.4, also see Chapter 5). In 1952, the biochemist Gunnar Blix introduced the term sialic acid from the Greek aaXia (salia) or saliva after isolating this molecule from the salivary gland mucin. Sialic acid is now the generic term for the family that includes derivatives with a C5-amino substituent called neuraminic acid. Neu5Ac (5) is the type of sialic acid found in humans [47],... 

The protein backbone cloaked by the oligosaccharides is rich in the amino acids serine, threonine and proline. The naked protein regions are characterized by a high cysteine content which provides the sulfur molecules for disulfide bonding. The wide range of sizes of respiratory mucins is believed to result from a variable number of sub-units forming the polymer. 

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