Of the thousands of known proteins, elastase is not one of the best known. However, it plays an important role as an enzyme, and much research is still produced on elastase. Its structure and sequence is known, and its function as an enzyme has been studied. This paper will describe the structure and function of this protein, its role as an enzyme, diseases associated with uninhibited elastase, and some interesting research involving elastase.
Elastase is an enzyme found especially in pancreatic juice that catalyzes the hydrolysis of elastin. Elastase is the main digester of elastin, which is the protein that makes up the elasticity in tissues. Elastin helps keep skin flexible but tight and stretches to accommodate normal activities like flexing a muscle. Elastase helps digest and break down this protein, which is a component of meat. However, when it is no longer controlled properly, elastase can cause some serious damage to its host. In this sense, elastase is possibly the most destructive enzyme having the ability to degrade virtually all of the connective components in the body. Uncontrolled proteolytic degradation by elastase has been implicated in a number of pathological conditions. (1).
There are several specific types of elastase, such as elastase 1 and 2. These have very similar structures, but serve a different function. The two main types in humans are pancreatic and neutrophil elastase. Pancreatic elastase is elastase 1, and it is responsible for catalyzing the breaking down of elastin. Neutrophil elastase, or elastase 2, breaks down bacteria during inflammation.
The elastase 1 structure researched for this paper has the PDB ID 3hgn. Its structure contains 240 residues. The structure of elastase 1 is relatively simple. It has been analyzed by high-resolution neutron and x-ray crystallography (1). There is one chain (A). The secondary structure contains both alpha helices and beta sheets. Figure 1 depicts the structure of PDB 3hgn.
Figure 1. The
Of Elastase (3hgn). (1).
As a typical enzyme, elastase has an active site. The active site is a cleft that pulls in the substrate to create an enzyme-substrate complex, which then leads to a product. Elastase is a serine protease and therefore has a structure that is designed to cleave proteins (specifically elastin). The structure of elastase is quite similar to some other serine proteases, such as chymotrypsin, which, according to a BLAST search, has about 40% identical sequence, and a very similar overall structure. These two proteins are obviously homologs, and are divergent in evolution. As a serine protease, elastase has an active site with three main amino acids: aspartate, histidine, and serine, which make up the catalytic triad that work together to create a nucleophilic catalysis. Elastase has bulky valine residues that aid in closing off the pocket of the cleaving site (2). The catalytic triad of nucleophilic elastase is shown in Figure 2, which illustrates the molecular model showing a substrate (yellow) bound to the active site of human neutrophil elastase (magenta). A substrate histidine (red) at the P2 but not P1 position can be virtually superimposed upon the catalytic histidine, H57 (green), and mimic the interactions of H57 with the other members of the catalytic triad, S195 and D102 (green). (3).
Figure 2: Catalytic triad (3)
Elastase operates like any enzyme would in that it catalyses the hydrolysis of a certain tissue. An enzyme is capable of speeding up this reaction so that it is complete in a matter of milliseconds, increasing the rate by a factor of about 1017 than with no enzyme. Elastase is a protease, meaning it breaks down a protein. According to Berg, elastase cleaves at the peptide bond after amino acids with small side chains. Elastase cleaves the peptide bonds in elastin, aiding in the digestibility of this elastic protein (2).
During the reaction in which elastase breaks down the substrate (elastin), two phases occur: a burst phase during which the amino side of the peptide bond is released, and a steady-state phase, in which the acyl side of the substrate is released. The elastin is positioned on the enzyme elastase so that the catalytic triad has access to the peptide bond. After this occurs, serine nucleophilically attacks the carbonyl of the peptide bond. A tetrahedral intermediate is formed and decomposed. Water enters the active site and attacks the reaction, causing the nitrogen terminus to leave. Water then attacks the acyl-enzyme intermediate and causes the release of the carboxylic acid component (2). Elastase is successful in breaking apart elastin, thus aiding in the digestion of this elastic protein.
Role in Research
Elastase has shown to be useful in medical science. According to research, such as Mastic’s article “Elastase-Dependent Live Attenuated Swine Influenza A Viruses Are Immunogenic and Confer Protection against Swine Influenza A Virus Infection in Pigs” has shown that elastase can play a role in modern immunizations against the most feared virus of today: H1N1, or commonly known as the “swine flu.” According to Mastic, Both NS1-truncated and elastase-dependent LAIV are weakened in swine and are immunogenic, meaning they produce an immune response. This protection was characterized by significantly reduced macroscopic and microscopic lung lesions, lower virus titers from the respiratory tract, and lower levels of proinflammatory cytokines. Thus, elastase-dependent Swine Influenze Virus (SIV) mutants can be used as live-virus vaccines against swine influenza in pigs (4). The two viruses were grown in the presence of elastase. Each live vaccine was tested on pigs. After the pigs were euthanized, their bodies were studied for signs of the influenza flu. The research showed that the two virus strains that were grown in the presence of elastase created a protection for the immune system to fight the H1N1 virus. (4). This shows that the immune response of elastase (the neutrophil elastase was used) can be advantageous to fighting off viruses. Although this particular research was performed on pigs, it may be possible to produce similar results for the human H1N1 virus vaccination. Immunizations with elastase have shown a better antibody response than other proteases and non-proteases. (5).
Elastase has also shown to be useful in the digestion of caseins in milk. A. Santillo’s article “Role of indigenous enzymes in proteolysis of casein in caprine milk,” evaluates the activity of the main indigenous proteolytic enzymes in caprine milk on hydrolysis of casein and to assess the potential significance of indigenous enzymes for quality of goat milk and dairy products. The article shows that serine proteases, which included elastase, were relevant to the hydrolysis of both α and β casein. Casein is a protein that is precipitated and used in the process of cheese making. This article showed that healthy milk contains enzymes, including elastase, to help break down caseins. Serine proteases (including elastase) may also be of significance to the production of potential bioactive peptides. (6).
Although elastase has these positive attributes, it also has its negative drawbacks. Elastase, when it is no longer functioning properly, can have some devastating effects. Elastase may become uncontrollable and begin to break down elastin in mass quantities in the body. This may lead to diseases such as emphysema--the loss of elasticity in the lungs. Degradation of the elastin network in diseases such as pulmonary emphysema or vessel wall aneurysms greatly affects the normal function of the tissues. Investigators have shown that the proteolysis of lung tissue with elastase mimics the destruction of elastin in emphysema. The main functional effect ofthis tissue destruction is a greatly decreased overall tissue stiffness. It is believed that uncontrolled elastase activity is responsible for the lung pathology associated with this disease. (7).
The reason for this is that the main inhibitor of elastase, alpha-1-antitrypsin (A1T1), becomes a deficiency due to things like tobacco smoke, which can damage A1T1 and prevent it from inhibiting elastase. In the article “Secretory Leukocyte Proteinase Inhibitor, Alpha-1-Antitrypsin Deficiency and Emphysema: Preliminary Study, Speculation and an Hypothesis” by Monna Ayad, four case studies were made with people who smoked or had A1T1 deficiency. The study showed that A1T1 deficiency could result from a genetic disorder or from smoking tobacco (7).
In conclusion, the importance of Elastase is part of our everyday lives. It aids in our digestion when we eat foods such as meat. It also furthers our understanding of how serine proteases operate. Elastase has been a useful component in finding vaccines for a virus that is causing global concern today. As useful as elastase can be, it can be harmful if not inhibited properly, causing diseases such as emphysema, in which elastase begins breaking down the elastin in tissues of its own host organism. Elastase does play a role in our lives, and is a protein worth knowing about.
Tamada T., K. T. . K. K. . A. M. . O. T. . I. K. . K. R. . T. T. Combined High-Resolution Neutron and X-ray Analysis of Inhibited Elastase Confirms the Active-Site Oxyanion Hole but Rules against a Low-Barrier Hydrogen Bond.. Journal of the American Chemical Society 2009, 131 (31), 11033-11040.
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Masic, A. Elastase-Dependent Live Attenuated Swine Influenza A Viruses Are Immunogenic and Confer Protection against Swine Influenza A Virus Infection in Pigs. Journal of virology 2009, 83 (19), 10198.
5. Darani, H. Y. . D. M. J. Anomalous Immunogenic Properties of Serine Proteases. Scandinavian Journal of Immunology 2009, 70 (4), 384-388.
Santillo, A. . K. A. L. . P. C. . S. A. . A. M. Role of indigenous enzymes in proteolysis of casein in caprine milk. International Dairy Journal 2009, 19 (11), 655-660.
7. Black L., K. K. B. S. M. M. Effects of elastase on the mechanical and failure properties of engineered elastase-rich matrices. J Appl Physiol 2005, 98, 1434-1441.