端木·宇 2008-6-19 22:24
Enzymes
Some chemical reactions simply happen when the two reactants come intocontact. For example, you may be familiar with the bubbly “volcano”that forms when baking soda and vinegar are placed together in a glass.This reaction is spontaneous because it does not require outside energyto force it to occur.
Most reactions, however, require energy.For example, the chemical reactions that produce a cake do not takeplace when baking soda, flour, and the other ingredients of a cake aresimply left in a pan on the kitchen counter. Heat is required to breakthe existing chemical bonds in the ingredients so that they can undergochemical reactions and combine with each other in new ways.
In the laboratory, chemists use heat tocreate the activation energy needed to get nonspontaneous reactionsstarted. Animals, however, can’t rely on internal Bunsen burners to gettheir chemical reactions cooking. In order to perform chemicalreactions at low temperatures, the body uses special proteins calledenzymes, which lower the activation energy necessary for chemicalreactions to achievable levels. Enzymes lower the activation energy byinteracting with the [b]substrates[/b], the primary molecules orcompounds involved in the reaction. If you think of the activationenergy needed for a chemical reaction as a mountain that the reactantshave to climb, think of an enzyme as opening up a tunnel through themountain. Less energy is required to go through the tunnel than toclimb all the way up the mountain.
Enzymes are not themselves altered whenthey help reactions along. Consequently, a single enzyme can be usedrepeatedly in many reactions. Because enzymes can be used over and overagain and because they can act very quickly, a relatively small amountof enzyme is needed to facilitate reactions involving relatively largeamounts of material.
Each enzyme is designed to fit only thesubstrates in the reaction that the enzyme is meant to control. Theone-to-one correspondence between enzyme and substrate is referred toas [b]specificity[/b]. An analogy to a [b]lock and key[/b] is usefulfor understanding the specificity of enzymes. Each enzyme can bethought of as a lock that can interact only with the appropriate key,or substrate. The region of the enzyme that interacts with thesubstrate is known as the [b]active site[/b].
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Enzymes help form bonds by holding twosubstrates near each other in the active site. Compounds can form bondswith each other more easily when they are adjacent than when they arefloating around the cell randomly.
Often, enzymes are named for theirsubstrate. The name of the enzyme is the name of the starting materialfollowed by the “-ase.” For example, maltase is an enzyme that breaksdown maltose, a common sugar. (Be careful not to confuse sugars, whichend in “-ose,” with enzymes, which end in “-ase.”)
[b] Factors Affecting Enzymes[/b]
Like all proteins, enzymes have a uniquethree-dimensional structure that changes under unusual environmentalconditions. Enzymes do not function well when their structure isaltered.
[i] Temperature and pH[/i]
Depending on where it is normally located inthe body, an enzyme will have different temperature and pH values atwhich its structure is most stable. As conditions deviate from thispoint, the enzyme’s ability to help along reactions decreases.
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Most enzymes work best near a pH of 7, butsome enzymes operate most effectively in a particularly acidicenvironment, such as the stomach; a neutral environment impairs theirfunction. Likewise, the enzymes of creatures that live at hightemperatures, such as bacteria that live in hot springs, do notfunction properly at human body temperature.
[i] Cofactors and Inhibitors[/i]
In order to control enzyme activity moreprecisely, the body has developed a number of compounds that turnenzymes on or off and make them work faster or slower. Sometimes thesecompounds attach to the active site along with the substrate, andsometimes they bind to another site on the enzyme. Activators ofenzymes are known as cofactors or [b]coenzymes[/b]. Many vitamins are coenzymes. Molecules that prevent enzymes from functioning properly are known as inhibitors.
