Extracellular posttranslational modification

Many other peptides are synthesized as proproteins that require modifications before attaining biologic activity. Many of the posttranslational modifications involve the removal of amino terminal amino acid residues by specific aminopeptidases. Collagen, an abundant protein in the extracellular spaces of higher eukaryotes, is synthesized as procollagen. Three procol-... 

All of the known class A receptors are subject to posttranslational modification at one or more iV-linked glycosylation sequences, found either in the extracellular amino terminns or in the second exdacellular loop. Glycosylation is required for the expression of some GPCRs at the plasma membrane (12,13). Furthermore, many receptors, snch as rhodopsin and the dopamine receptors, are also subject to other posttranslational modifications, such as palmitoylation at the intracellular domains... 

Tropoelastin molecules are crosslinked in the extracellular space through the action of the copper-dependent amine oxidase, lysyl oxidase. Specific members of the lysyl oxidase-like family of enzymes are implicated in this process (Liu etal, 2004 Noblesse etal, 2004), although their direct roles are yet to be demonstrated enzymatically. Lysyl oxidase catalyzes the oxidative deamination of e-amino groups on lysine residues (Kagan and Sullivan, 1982) within tropoelastin to form the o-aminoadipic-6-semialdehyde, allysine (Kagan and Cai, 1995). The oxidation of lysine residues by lysyl oxidase is the only known posttranslational modification of tropoelastin. Allysine is the reactive precursor to a variety of inter- and intramolecular crosslinks found in elastin. These crosslinks are formed by nonenzymatic, spontaneous condensation of allysine with another allysine or unmodified lysyl residues. Crosslinking is essential for the structural integrity and function of elastin. Various crosslink types include the bifunctional crosslinks allysine-aldol and lysinonorleucine, the trifunctional crosslink merodes-mosine, and the tetrafunctional crosslinks desmosine and isodesmosine (Umeda etal, 2001). 

N is often limiting in the marine environment. Further, many enzymes are sensitive to cellular substrate concentrations rather than extracellular concentrations and it is difficult to measure the relevant intracellular metabohte pools. In vitro assays may affect the conformation of enzymes and the degree to which they are modified. For example, allosteric effects (see Section 1.3.3) may be modified under in vitro conditions. Many enzymes undergo posttranslational regulation wherein enzyme activity is affected by binding of activator/inactivator proteins and covalent modification of the enzyme (e.g., adenylylation, phosphorylation or carbamylation) (Ottaway, 1988). When there is posttranslational modification of enzymes, enzyme activity measured in assays may be unrelated to in vivo activity (see Section 2.2.1) and there are few ways to determine the extent of enzyme modification in nature. 

Protein synthesis is an extraordinarily complex process in which genetic information encoded in the nucleic acids is translated into the 20 amino acid alphabet of polypeptides. In addition to translation (the mechanism by which a nucleotide base sequence directs the polymerization of amino acids), protein synthesis can also be considered to include the processes of posttranslational modification and targeting. Posttranslational modification consists of chemical alterations cells use to prepare polypeptides for their functional roles. Several modifications assist in targeting, which directs newly synthesized molecules to a specific intracellular or extracellular location. 

Posttranslational modification reactions prepare polypeptides to serve their specific functions and direct them to specific cellular or extracellular locations. Examples of these modifications include proteolytic processing (e.g., removal of signal proteins), glycosylation, methylation, phosphorylation, hydroxylation, lipophilic modifications (e.g., N-myristoylation and prenylation), and disulfide bond formation. 

Source of antigen - The nature of the injected antigen (protein, peptide from a specific sequence) and the species of the antigen. Sometimes antibodies to specific parts of molecules are needed (e.g., the extracellular domain, a specific sequence of amino acids or a posttranslational modification). 

Lysine tyrosylquinone (LTQ) (Figure 3) is the protein-derived cofactor of mammalian lysyl oxidase, an important enzyme in the metabolism of connective tissue. Lysyl oxidase catalyzes the posttranslational modification of elastin and collagen. It oxidizes selected peptidyl lysine residues to peptidyl a-aminoadipic -semialdehyde residues. This initiates formation of the covalent cross-linkages that insolubilize these extracellular proteins. This enzyme also contains copper as a second prosthetic group. 

Collagen, a family of fibrous proteins, is produced by a variety of cell types but principally by fibroblasts (cells found in interstitial connective tissue), muscle cells, and epithelial cells. Type I collagen [collagen(l)], the most abundant protein in mammals, is a fibrous protein that is the major component of connective tissue. It is found in the extracellular matrix (ECM) of loose connective tissue, bone, tendons, skin, blood vessels, and the cornea of the eye. Collagen(l) contains approximately 33% glycine and 21% proline and hydroxyproline. Hydroxyproline is an amino acid produced by posttranslational modification of peptidyl proline residues (see Chapter 7, section V.C., for an earlier introduction to collagen). 

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