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Many chemical reactions occur within living cells. Most of these reactions happen too slowly to be biologically relevant. A special class of protein molecules called enzymes (in ferment) speed up such reactions. Chemical reactions need a certain amount of [activation energy]? to take place. Enzymes can increase reaction speed by allowing a different reaction path with a lower activation energy (Fig. 1), making it easier for the reaction to occur. Enzymes are large proteins that catalyze (accelerate) chemical reactions. They are essential for the function of cells. Enzymes are very specific to the reactions they catalyze, and the chemicals (substrates) that are involved in the reactions. Substrates fit their enzymes like a key fits its lock (Fig. 2). Many enzymes are composed of several proteins that act together as a unit. Most parts of an enzyme have regulatory or structural purposes. The catalyzed reaction takes place in only a small part of the enzyme called the [active site]?. |
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Many chemical reactions occur within biological cells. Most of these reactions happen too slowly to be biologically relevant. A special class of protein molecules called enzymes (in ferment) speed up such reactions. Chemical reactions need a certain amount of [activation energy]? to take place. Enzymes can increase reaction speed by allowing a different reaction path with a lower activation energy (Fig. 1), making it easier for the reaction to occur. Enzymes are large proteins that catalyze (accelerate) chemical reactions. They are essential for the function of cells. Enzymes are very specific to the reactions they catalyze, and the chemicals (substrates) that are involved in the reactions. Substrates fit their enzymes like a key fits its lock (Fig. 2). Many enzymes are composed of several proteins that act together as a unit. Most parts of an enzyme have regulatory or structural purposes. The catalyzed reaction takes place in only a small part of the enzyme called the [active site]?. |
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There are a number of factors that can influence the reaction speed, catalytic activity, and specificity of an enzyme. Apart from [de novo]? synthesis (the production of more enzymes to increase catalysis rates), basic factors of the medium inside the cell like pH value or temperature can denaturate an enzyme (alter its shape) so that it can no longer function. More specific regulation is possible by [posttranslational modification]? (e.g., phosphorylation?) of the enzyme or by adding cofactor?s like metal ions or organic molecules (e.g. NAD+, FAD, CoA? or vitamins) that interact with the enzyme. Allosteric? enzymes are composed of several subunits (proteins) that interact with each other and thus influence each other's catalytic activity. In addition to that, enzymes can be regulated by competitive inhibitors (Fig. 4) and uncompetitive inhibitors and activators (Fig. 5). Inhibitors and activators are often used as medicines, but they can also be poisonous. |
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There are a number of factors that can influence the reaction speed, catalytic activity, and specificity of an enzyme. Apart from [de novo]? synthesis (the production of more enzymes to increase catalysis rates), basic factors of the medium inside the cell like pH value or temperature can denaturate an enzyme (alter its shape) so that it can no longer function. More specific regulation is possible by [posttranslational modification]? (e.g., phosphorylation?) of the enzyme or by adding cofactor?s like metal ions or organic molecules (e.g. NAD?+, FAD?, CoA? or vitamins) that interact with the enzyme. Allosteric? enzymes are composed of several subunits (proteins) that interact with each other and thus influence each other's catalytic activity. In addition to that, enzymes can be regulated by competitive inhibitors (Fig. 4) and uncompetitive inhibitors and activators (Fig. 5). Inhibitors and activators are often used as medicines, but they can also be poisonous. |
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Figure 4: Competitive inhibition. |
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Figure 4: Competitive inhibition. |
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</font> substrate (S), sometimes even better. The inhibitor (I) takes the place of the substrate (S) in the active center, but cannot undergo the catalytic reaction, thus inhibiting the enzyme (E) from catalyzing a substrate (S) molecule. Some inhibitors (I) build covalent bonds with the enzyme (E), deactivating it permanently ([suicide inhibitors]?). |
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</font> substrate (S), sometimes even better. The inhibitor (I) takes the place of the substrate (S) in the active center, but cannot undergo the catalytic reaction, thus inhibiting the enzyme (E) from catalyzing a substrate (S) molecule. Some inhibitors (I) build covalent bonds with the enzyme (E), deactivating it permanently ([suicide inhibitors]?). |
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Figure 5: Uncompetitive inhibition. |
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http://www.nupedia.com/newsystem/upload_file/795/ncomp_inhib.png Figure 5: Uncompetitive inhibition. |
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http://www.nupedia.com/newsystem/upload_file/795/pathway.png Figure 6: Diagram of a metabolic pathway. The substrate (S) is catalyzed by an enzyme (E1) to an intermediate product (I1), which is then further catalyzed by several enzymes to the final product (P). The product then inhibits the first enzyme of the pathway, thus regulating the amount of product created. |
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Figure 7: Common feedback inhibition mechanisms. |
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Figure 6: Common feedback inhibition mechanisms. |
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Enzymes are essential to living organisms, and a malfunction of even a single enzyme can lead to severe or lethal sickness. An example for a sickness caused by an enzyme malfunction in humans is phenylketonuria. The enzyme [phenylalanine hydroxylase]? that usually catalyzes the essential amino acid phenylalanine into tyrosine does not work, resulting in a buildup of phenylalanine that leads to mental retardation. Enzymes in the human body can also be influenced by inhibitors in either good or bad ways. Aspirin, for example, inhibits an enzyme that produces prostaglandins (inflammation messengers), thus suppressing pain. Carbon monoxide (CO) is a competitive inhibitor (Fig. 4) for oxygen (O2) when binding to hemoglobin. Carbon monoxide has a 230-270x affinity for hemoglobin than oxygen and therefore displaces oxygen. Enzymes are also used in everyday products such as biological washing detergents. |
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Enzymes are essential to living organisms, and a malfunction of even a single enzyme can lead to severe or lethal sickness. An example for a sickness caused by an enzyme malfunction in humans is phenylketonuria. The enzyme [phenylalanine hydroxylase]? that usually catalyzes the essential amino acid phenylalanine into tyrosine does not work, resulting in a buildup of phenylalanine that leads to mental retardation. Enzymes in the human body can also be influenced by inhibitors in either good or bad ways. Aspirin, for example, inhibits an enzyme that produces prostaglandin?s (inflammation messengers), thus suppressing pain. Carbon monoxide (CO) is a competitive inhibitor (Fig. 4) for oxygen (O2) when binding to hemoglobin. Carbon monoxide has a 230-270x affinity for hemoglobin than oxygen and therefore displaces oxygen. Enzymes are also used in everyday products such as biological washing detergents. |
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By common convention, an enzyme's name consists of a description of what it does, with the word ending "-ase" added; examples are [alcohol dehydrogenase]?, [DNA polymerase]? |
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By common convention, an enzyme's name consists of a description of what it does, with the word ending "-ase" added; examples are [alcohol dehydrogenase]? and [DNA polymerase]?. |
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