Information AboutEnzyme |
| CATEGORIES ABOUT ENZYME | |
| biomolecules | |
| enzymes | |
| metabolism | |
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Enzymes are Protein s that Catalyze (''i.e.'' Accelerate ) Chemical Reaction s.Smith AD (Ed) ''et. al.'' (1997) ''Oxford Dictionary of Biochemistry and Molecular Biology'' Oxford University Press ISBN 0-19-854768-4 In enzymatic reactions, the Molecule s at the beginning of the process are called Substrate s, and the enzyme converts them into different molecules, the products. Almost all processes in a Biological Cell need enzymes in order to occur at significant rates. Since enzymes are extremely selective for their substrates and speed up only a few reactions from among many possibilities, the set of enzymes made in a cell determines which Metabolic Pathway s occur in that cell. Like all catalysts, enzymes work by lowering the Activation Energy (''E''a or Δ''G''‡) for a reaction, thus dramatically accelerating the rate of the reaction. Most enzyme reaction rates are millions of times faster than those of comparable uncatalyzed reactions. As with all catalysts, enzymes are not consumed by the reactions they catalyze, nor do they alter the Equilibrium of these reactions. However, enzymes do differ from most other catalysts by being much more specific. Enzymes are known to catalyze about 4,000 biochemical reactions.1 Although all enzymes are proteins, not all biochemical catalysts are enzymes, since some RNA molecules called Ribozyme s also catalyze reactions.2 Synthetic molecules called Artificial Enzyme s also display enzyme-like catalysis.3 Enzyme activity can be affected by other molecules. Inhibitors are molecules that decrease enzyme activity; Activator s are molecules that increase activity. Many Drug s and Poison s are enzyme inhibitors. Activity is also affected by Temperature , chemical environment (e.g. PH ), and the Concentration of substrate. Some enzymes are used commercially, for example, in the synthesis of Antibiotic s. In addition, some household products use enzymes to speed up biochemical reactions (''e.g.'', enzymes in biological Washing Powder s break down protein or Fat stains on clothes; enzymes in Meat Tenderizer s break down proteins, making the meat easier to chew). ETYMOLOGY AND HISTORY ]] As early as the late 1700s and early 1800s , the digestion of Meat by stomach secretions4 and the conversion of Starch to Sugar s by plant extracts and Saliva were known. However, the mechanism by which this occurred had not been identified.Williams, H. S. (1904) [http://etext.lib.virginia.edu/toc/modeng/public/Wil4Sci.html A History of Science: in Five Volumes. Volume IV: Modern Development of the Chemical and Biological Sciences] Harper and Brothers (New York) Accessed 04 April 2007 In the 19th century, when studying the Fermentation of sugar to Alcohol by Yeast , Louis Pasteur came to the conclusion that this fermentation was catalyzed by a vital force contained within the yeast cells called " Ferments ", which were thought to function only within living organisms. He wrote that "alcoholic fermentation is an act correlated with the life and organization of the yeast cells, not with the death or putrefaction of the cells."5 In 1878 German physiologist '', which comes from Greek ''ενζυμον'' "in leaven", to describe this process. The word ''enzyme'' was used later to refer to nonliving substances such as Pepsin , and the word ''ferment'' used to refer to chemical activity produced by living organisms. In 1897 "for his biochemical research and his discovery of cell-free fermentation". Following Buchner's example; enzymes are usually named according to the reaction they carry out. Typically the suffix ''-ase'' is added to the name of the Substrate (''e.g.'', Lactase is the enzyme that cleaves Lactose ) or the type of reaction (''e.g.'', DNA Polymerase forms DNA polymers). Having shown that enzymes could function outside a living cell, the next step was to determine their biochemical nature. Many early workers noted that enzymatic activity was associated with proteins, but several scientists (such as Nobel laureate Richard Willstätter ) argued that proteins were merely carriers for the true enzymes and that proteins ''per se'' were incapable of catalysis. However, in 1926, James B. Sumner showed that the enzyme Urease was a pure protein and crystallized it; Sumner did likewise for the enzyme Catalase in 1937. The conclusion that pure proteins can be enzymes was definitively proved by Northrop and Stanley , who worked on the digestive enzymes pepsin (1930), trypsin and chymotrypsin. These three scientists were awarded the 1946 Nobel Prize in Chemistry. 1946 Nobel prize for Chemistry laureates at http://nobelprize.org Accessed 04 April 2007 This discovery that enzymes could be crystallized eventually allowed their structures to be solved by X-ray Crystallography . This was first done for Lysozyme , an enzyme found in tears, saliva and Egg White s that digests the coating of some bacteria; the structure was solved by a group led by David Chilton Phillips and published in 1965.6 This high-resolution structure of lysozyme marked the beginning of the field of Structural Biology and the effort to understand how enzymes work at an atomic level of detail. STRUCTURES AND MECHANISMS See Also: Enzyme catalysis . The grey sphere is the Enzymes are proteins, and range from just 62 amino acid residues in size for the . Enzymes can also contain sites that bind Cofactors , which are needed for catalysis. Some enzymes also have binding sites for small molecules, which are often direct or Indirect products or substrates of the reaction catalyzed. This binding can serve to increase or decrease the enzyme's activity, providing a means for Feedback regulation. Like all proteins, enzymes are made as long, linear chains of amino acids that Fold to produce a Three-dimensional Product . Each unique amino acid sequence produces a specific structure, which has unique properties. Individual protein chains may sometimes group together to form a Protein Complex . Most enzymes can be Denatured —that is, unfolded and inactivated—by heating, which destroys the Three-dimensional Structure of the protein. Depending on the enzyme, denaturation may be reversible or irreversible. Specificity Enzymes are usually very specific as to which reactions they catalyze and the Substrate s that are involved in these reactions. Complementary shape, charge and Hydrophilic / Hydrophobic characteristics of enzymes and substrates are responsible for this specificity. Enzymes can also show impressive levels of Stereospecificity , Regioselectivity and Chemoselectivity .10 Some of the enzymes showing the highest specificity and accuracy are involved in the copying and expression of the Genome . These enzymes have "proof-reading" mechanisms. Here, an enzyme such as DNA Polymerase catalyses a reaction in a first step and then checks that the product is correct in a second step.11 This two-step process results in average error rates of less than 1 error in 100 million reactions in high-fidelity Mammalian polymerases.Berg J., Tymoczko J. and Stryer L. (2002) ''Biochemistry.'' W. H. Freeman and Company ISBN 0-7167-4955-6 Similar proofreading mechanisms are also found in RNA Polymerase ,12 Aminoacyl TRNA Synthetase s13 and Ribosome s.14 Some enzymes that produce Secondary Metabolite s are described as promiscuous, as they can act on a relatively broad range of different substrates. It has been suggested that this broad substrate specificity is important for the evolution of new biosynthetic pathways.15 "Lock and key" model |
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