Extracellular posttranslational

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-... 

Advantages Posttranslational modihcation of protein product can be performed extracellular expression of proteins requires less complex purihcation processes. 

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... 

Slobodyansky E, Berkovich A, Bovolin P, et al The endogenous allosteric modulation of GABA-A receptor subtypes a role for the neuronal posttranslational processing products of rat brain DBl, in GABA and Benzodiazepine Receptor Subtypes. Edited by Biggio G, Costa E. New York, Raven, 1990, p 52 Small DH, Nurcombe V, Moir R Association and release of the amyloid protein precursor of Alzheimer s disease from chick brain extracellular matrix. J Neurosci 12 4143-4150, 1992... 

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). 

In the analysis of proteins in forensic applications, the chemical modifications that occur to proteins, posttranslationally and nonenzymatically, are of primary importance. These chemical changes are a result of chemical reactions between side chains of the protein and reactive groups of metabolites and/or exogenous toxicants, including drugs present in extracellular fluid such as serum. The analytical accessibility of these modified proteins depends on their rate of turnover. For example, those with a slow turnover rate will be long-lived, and such problems will be much more easy to identify than those with faster turnover rates. 

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. 

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