Exoenzyme
An exoenzyme, or extracellular enzyme, is an enzyme that is secreted by a cell and functions outside of that cell. Exoenzymes are produced by both prokaryotic and eukaryotic cells and have been shown to be a critical component of many biological processes. Most often these enzymes are involved in the breakdown of larger macromolecules. The breakdown of these larger macromolecules is critical for allowing their constituents to pass through the cell membrane and enter into the cell. For humans and other complex organisms, this process is best characterized by the digestive system which breaks down solid food [1] via exoenzymes. The small molecules, generated by the breakdown, enter into cells and are utilized for various cellular functions. In addition to this integral role in biological systems, different classes of microbial exoenzymes have been used by humans since pre-historic times for such diverse purposes as food production, biofuels, textile production and in the paper industry.[2] Another important role that microbial exoenzymes serve is in the natural ecology and bioremediation of terrestrial and marine[3] environments.
History
Very limited information is available about the original discovery of exoenzymes. According to Merriam-Webster dictionary, the term "exoenzyme" was first recognized in the English language in 1908.[4] The book "Intracellular Enzymes: A Course of Lectures Given in the Physiological," by Horace Vernon is thought to be the first publication using this word in that year.[5] Based on the book, it can be assumed that the first known exoenzymes were pepsin and trypsin, as both are mentioned by Vernon to have been discovered by scientists Briike and Kiihne before 1908.[6]
Examples of Exoenzymes
Amylases
Amylases are a group of extracellular enzymes (glycoside hydrolases) that catalyze the hydrolysis of starch. These enzymes are grouped into three classes based on their amino acid sequences, mechanism of reaction, method of catalysis and their structure.[7] The different classes of amylases are α-amylases, β-amylases, and glucoamylases. The α-amylases hydrolyze starch by randomly cleaving the 1,4-a-D-glucosidic linkages between glucose units, β-amylases cleave non-reducing chain ends of components of starch such as amylose, and glucoamylases hydrolyze glucose molecules from the ends of amylose and amylopectin.[8] Amylases are critically important extracellular enzymes and are found in plants, animals and micro-organisms. In humans, amylases are secreted by both the pancreas and salivary glands with both sources of the enzyme required for complete starch hydrolysis [9].
Lipoprotein lipase
Lipoprotein lipase (LPL) is a type of digestive enzyme that helps regulate the uptake of triacylglycerols from chylomicrons and other low-density lipoproteins from fatty tissues in the body.[10] The exoenzymatic function allows it to break down the triacylglycerol into two free fatty acids and one molecule of monoacylglycerol. LPL can be found in endothelial cells in fatty tissues, such as adipose, cardiac, and muscle.[10] Lipoprotein lipase is downregulated by high levels of insulin,[11] and upregulated by high levels of glucagon and adrenaline.[10]
Pectinase
Pectinases, also called pectolytic enzymes, are a class of exoenzymes that are involved in the breakdown of pectic substances, most notably pectin.[12] Pectinases can be classified into two different groups based on their action against the galacturonan backbone of pectin: de-esterifying and depolymerizing.[13] These exoenzymes can be found in both plants and microbial organisms including fungi and bacteria.[14] Pectinases are most often used to breakdown the pectic elements found in plants and plant derived products.
Pepsin
Discovered in 1836, pepsin was one of the first enzymes to be classified as an exoenzyme.[6] The enzyme is first made in the inactive form, pepsinogen by chief cells in the lining of the stomach.[15] With an impulse from the vagus nerve, pepsinogen is secreted into the stomach, where it mixes with hydrochloric acid to form pepsin.[16] Once active, pepsin works to break down proteins in foods such as dairy, meat, and eggs.[15] Pepsin works best at the pH of gastric acid, 1.5 to 2.5, and is deactivated when the acid is neutralized to a pH of 7.[15]
Also one of the first exoenzymes to be discovered, trypsin was named in 1876, forty years after pepsin.[17] This enzyme is responsible for the breakdown of large globular proteins and its activity is specific to cleaving the C-terminal sides of arginine and lysine amino acid residues[17] . It is the derivative of trypsinogen, an inactive precursor that is produced in the pancreas[18] . When secreted into the small intestine, it mixes with enterokinase to from active trypsin. Due to its role in the small intestine, trypsin works at an optimal pH of 8.0[19] .
Biotechnological and Industrial Applications
Microbiological sources of exoenzymes including amylases, proteases, pectinases, lipases, xylanases, cellulases among others are used for a wide range of biotechnological and industrial uses including biofuel generation, food production, paper manufacturing, detergents and textile production [2]. Optimizing the production of biofuels has been a focus of researchers in recent years and is centered around the use of microorganisms to convert biomass into ethanol. The enzymes that are of particular interest in ethanol production are cellobiohydrolase which solubilizes crystalline cellulose and xylanase that hydrolyzes xylan into xylose [20]. One model of biofuel production is the use of a mixed population of bacterial strains or a consortium that work to facilitate the breakdown of cellulose materials into ethanol by secreting exoenzymes such as cellulases and laccases [20]. In addition to the important role it plays in biofuel production, xylanase is utilized in a number of other industrial and biotechnology applications due to it's ability to hydrolyze cellulose and hemicellulose. These applications include the breakdown of agricultural and forestry wastes, working as a feed additive to facilitate greater nutrient uptake by livestock, and as an ingredient in bread making to improve the rise and texture of the bread [21].
Lipases are one of the most used exoenzymes in biotechnology and industrial applications. Lipases make ideal enzymes for these applications because they are highly selective in their activity, they are readily produced and secreted by bacteria and fungi, their crystal structure is well characterized, they do not require cofactors for their enzymatic activity, and they do not catalyze side reactions [22]. The range of uses of lipases encompasses production of biopolymers, generation of cosmetics, use as a herbicide, and as an effective solvent [22]. However, perhaps the most well known use of lipases in this field is it's use in the production of biodiesel fuel. In this role, lipases are used to convert vegetable oil to methyl- and other short-chain alcohol esters by a single transesterification reaction [23].
See also
References
- ^ Kong, F. (1 June 2008). "Disintegration of Solid Foods in Human Stomach". Journal of Food Science. 73 (5): R67–R80. doi:10.1111/j.1750-3841.2008.00766.x.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ a b Thiel, ed. by Joachim Reitner, Volker. Encyclopedia of geobiology. Dordrecht: Springer. pp. 355–359. ISBN 9781402092121.
{{cite book}}:|first=has generic name (help)CS1 maint: multiple names: authors list (link) - ^ Arnosti, Carol (15 January 2011). "Microbial Extracellular Enzymes and the Marine Carbon Cycle". Annual Review of Marine Science. 3 (1): 401–425. doi:10.1146/annurev-marine-120709-142731.
- ^ "Merriam-Webster". Retrieved 2013-10-26.
- ^ "Lexic.us". Retrieved 2013-10-26.
- ^ a b Vernon, Horace. "Intracellular Enzymes: A Course of Lectures Given in the Physiological". Retrieved 2013-10-26.
- ^ Sharma, Archana (2013). "Microbial acid-stable alpha-amylases: Characteristics, genetic engineering and applications". Process Biochemistry. 48: 201–211.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Pandey, Ashok (2000). "Advances in microbial amylases". Biotechnology Applied Biochemistry. 31: 135–152.
{{cite journal}}: C1 control character in|coauthors=at position 13 (help); Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Pandol, Stephen. "The Exocrine Pancreas". The Exocrine Pancreas. Morgan & Claypool Life Sciences. Retrieved 25 November 2013.
- ^ a b c Mead, JR (2002 Dec). "Lipoprotein lipase: structure, function, regulation, and role in disease". Journal of molecular medicine (Berlin, Germany). 80 (12): 753–69. PMID 12483461.
{{cite journal}}: Check date values in:|date=(help); Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Kiens, B (1989 Oct). "Effects of insulin and exercise on muscle lipoprotein lipase activity in man and its relation to insulin action". The Journal of clinical investigation. 84 (4): 1124–9. PMID 2677048.
{{cite journal}}: Check date values in:|date=(help); Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Jayani, Ranveer Singh (1 September 2005). "Microbial pectinolytic enzymes: A review". Process Biochemistry. 40 (9): 2931–2944. doi:10.1016/j.procbio.2005.03.026.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Alimardani-Theuil, Parissa (1 August 2011). "Yeasts: An attractive source of pectinases—From gene expression to potential applications: A review". Process Biochemistry. 46 (8): 1525–1537. doi:10.1016/j.procbio.2011.05.010.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Gummadi, Sathyanarayana N. (1 February 2003). "Purification and biochemical properties of microbial pectinases—a review". Process Biochemistry. 38 (7): 987–996. doi:10.1016/S0032-9592(02)00203-0.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ a b c "Encyclopedia Britannica". Retrieved November 14, 2013.
- ^ Guldvog, I (1981). "Physiological stimulation of pepsin secretion. The role of vagal innervation". Scandinavian journal of gastroenterology. 16 (1): 17–25. PMID 6785873.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ a b Worthington, Krystal. "Trypsin". Worthington Biochemical Corporation. Retrieved 26 November 2013.
- ^ "Trypsin". Free Dictionary. Retrieved 26 November 2013.
- ^ "Trypsin Product Information". Worthington Biochemical Corporation. Retrieved 26 November 2013.
- ^ a b Alper, Hal (1 October 2009). "Engineering for biofuels: exploiting innate microbial capacity or importing biosynthetic potential?". Nature Reviews Microbiology. 7 (10): 715–723. doi:10.1038/nrmicro2186.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Juturu, Veeresh (1 November 2012). "Microbial xylanases: Engineering, production and industrial applications". Biotechnology Advances. 30 (6): 1219–1227. doi:10.1016/j.biotechadv.2011.11.006.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ a b Jaeger, Karl-Erich (2002). "Lipases for biotechnology". Current Opinion in Biotechnology. 13: 390–397.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Fan, Xiaohu (2012). "Lipases as biocatalyst for biodiesel production". Methods Molecular Biology. 861: 471–483.
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