Elastin proteolysis

Emphysema. Emphysema is one disease where the linkage between proteolysis and the disease is fairly well understood. Pulmonary emphysema is a disease characterized by a progressive loss of lung elasticity due to the destruction of lung elastin and alveoli. Respiration becomes increasingly difficult and death often results. 

Figure 2. Synthesis of mature elastin fibers. Some evidence suggests the possibility for proforms to elastin that appear as the first products of translation. These products are cleaved to tropoelastin (27), which appears to combine with microfibrillar protein. Although post-translational events important to the synthesis of the microfibrillar protein have not been defined, it is clear that it is a major component on which is organized or assembled the profibrillar forms of elastin. Cross-linking is catalyzed by lysyl oxidase, a copper-requiring protein (30). Recent information on the elastin proteinase(s) involved in tropoelastolysis would suggest that proteolysis may also play a role in elastin fiber...

In particular, excessive proteolysis of elastin by HLE has been implicated in pulmonary emphysema [19]. In this case, the imbalance appears to result from reduced levels of active extracellular alpha,-proteinase inhibitor (a,-PI), the primary plasma inhibitor of HLE. This decrease is caused either by a genetic disorder (PiZZ phenotype individuals) or by reduction in the elastase inhibitory capacity (EIC) of ai-PI due to its oxidative inactivation by tobacco smoke [20]. The detailed evidence supporting the potential role of elastase in the development of emphysema has been extensively reviewed [21] and will not be repeated here. The fact that HLE is also a potent secretagogue [22] may play a role in several disease states, including cystic fibrosis [23], chronic bronchitis [24], and acute respiratory distress syndrome (ARDS) [25]. The mechanism of the secretagogue activity is not known, but, since the HLE-induced secretion can be blocked by specific HLE inhibitors, it appears to require catalytic activity by the enzyme [26]. 

It is not always possible to apply enzymatic hydrolysis directly to proteins as they are in the native form. Native, globular proteins (e.g., from soy, corn, almond) or fibrous insoluble proteins (e.g., collagen, keratins, elastin) are generally resistant to proteolysis this is generally explained by the compact tertiary structure of the protein that protects most of the peptide bonds. In the denatured, unfolded form the peptide bonds are exposed and available for enzymatic cleavage. As native proteins in aqueous solution are in dynamic equilibrium with a number of more or less distorted forms, part of which can be considered denatured and thereby accessible to enzyme attack, the initial break of a few peptide bonds can destabilize the protein molecule and cause irreversible unfolding in some cases (e.g., hydrolysis of egg albumin by pepsin) this mechanism allows the protease to perform the hydrolysis to a remarkable extent. More frequently, especially when covalent bonds (disulfide bonds) stabilize the native form of the protein, a preliminary partial or extended denaturation is needed to make enzymatic hydrolysis possible this is normally achieved by heating or chemical attack, or a combination of the two. 

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