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It is important to note that, as with all catalysts, all reactions catalyzed by enzymes must be 'spontaneous' (containing a net negative Gibbs free energy), i.e. with the enzyme, they run in the same direction as they would without the enzyme, just more quickly; the concept is similar to the likelihood of a ball rolling down a hill versus the likelihood of it rolling up the hill. This is required by the Law of Conservation of Energy, which would be violated by the possibility of a cycle of moving down a pathway releasing less net energy and back up a different pathway with higher net energy, or vice versa. Given a particular starting set of conditions, the end products of a particular reaction (including net energy), once steady state is reached, must always be identical, independent of the specific individual pathway taken from beginning point to end point. An enzyme can, however, run a normally nonspontaneous reaction 'backwards' by coupling it to a spontaneous one , as long as the net free energy from the total of both reactions is negative.
Enzymes are necessary because within biological cells, many chemical reactions would occur too slowly to sustain life; oxidation of organic food compounds to provide energy, for instance. Enzymes speed up reactions by a factor of one thousand times or more. They also provide a means to control the reactions, by modulating enzymatic activity.
An enzyme can be a monomeric protein made up of about hundred amino acids or more, or an oligomeric protein consisting of several monomers, different or identical, that act together as a unit. As with any protein, each monomer is actually produced as a long, linear chain of amino acids, which then folds up in a particular fashion to produce the correct three-dimensional product. The factors that go into ensuring that the chain folds correctly and maintains its shape are complex, and still not completely identified, although the general principles seem to be understood.
As a consequence of this basic structure, the amino acid chain of an enzyme tends to consist of one or more active regions, separated by stretches whose purpose is mainly to position the active regions correctly. Because the precise structure of each region tends to be fairly critical to correct function, and because the frequency of a mutation which would produce a nonfunctional active region is proportional to the length of the chain separating the amino acids involved, evolution works against having the active amino acids from an active region dispersed along the protein chain, instead tending to keep the amino acids involved in each active region compacted fairly close together in the chain and tightly folded, separating these regions by long stretches of 'spacer' amino acids where mutation is not critical (although exceptions occur). This has the additional effect of making each region act, relative to mutation, somewhat like an independent subunit which as a unit can be duplicated, deleted, moved, or 'mixed-and-matched' with other such regions, generating new proteins to be tested for evolutionary success. This would seem to be a more efficient process in terms of successfully generating new, functional, enzymes than would having the functionally related amino acids of the active site dispersed throughout the chain, with random mutation occurring anywhere in the amino acid chain frequently disrupting the function of the active site.
An enzyme contains an active site, consisting of the catalytic amino acids and one or more binding sites that bind the substrateThe word substrate can mean the following: In biochemistry, a substrate is a molecule undergoing a reaction, for which the presence of an enzyme lowers the activation energy. The substrate binds with the enzyme's active site, and the enzyme provides an al(s). Enzymes also frequently have binding sites that serve regulatory functions, which increase or inhibit the enzyme's activity. These typically bind metals or small molecules.
Enzymes can couple two or more reactions, so that a thermodynamically favourable reaction can be used to "drive" a thermodynamically unfavorable one, in effect running the reaction 'backwards', or 'uphill'. For instance, the high energy compound ATPFor other uses of the initials ATP, see ATP (disambiguation Adenosine triphosphate ATP is the nucleotide known in biochemistry as the " molecular currency" of intracellular energy transfer; that is, ATP is able to store and transport chemical energy withi is generated in the cell by coupling its synthesis to the oxidation of sugarThis article deals with sugar as food and as an important, widely traded commodity; the word also has other uses; see Sugar (disambiguation A sugar is a form of carbohydrate; the most commonly used sugar is a white crystalline solid, sucrose; used to altes, which releases more energy; then the ATP is broken down by other enzymes, releasing the energy stored in it in order to drive another, otherwise energetically unfavorable, chemical reaction. This is analogous to raising a weight by attaching it to a rope that goes over a pulley and attaches to a heavier weight which is then lowered; the net energy balance of the sum of both actions is favorable.
In a very similar fashion, an enzyme can cause a reaction to become more unbalanced in one direction, by coupling it to a different, very unbalanced reaction. As with any catalyst, an enzymatic pathway does not intrinsically change the reversibility or irreversibility of a reaction. The direction of any chemical reaction is based only on the energy balance between the two sides of the reaction equation, and if there is very little change in net free energy between the substrates and the products, the reaction will normally be easily reversible, running in either direction depending on which side of the reaction was more highly concentrated. However, enzymes provide a means to make such a reaction much less reversible, by coupling the reaction to one with a large release of energy. This is analogous to an object sliding freely on a flat surface; if it is desired that the object be pulled in one direction, it may be attached to a spring, or a rope attached to a hanging weight. In order to lower the total net energy, the system will drive the coupled reaction in one direction.