AP Biology, Block 4
October 18, 2012
Metabolism is the totality of all of an organism’s chemical reactions. Chemical reactions occur due to enzymes, a substance which acts as a catalyst in driving chemical reactions in order to produce a desired product (Campbell and Reece, 2002). A catalyst is usually a protein; however, some catalytic molecules counter this generalization. A discovery made in the early nineteen- nineties revealed that ribozymes, molecules made of ribonucleic acid (RNA), act as a catalyst in the transformation of an RAN molecule. Scientists concluded from the new discovery that the informational molecule RNA may have once been able to function without proteins (Dousti, 1995). However, enzymes are strictly proteins, and thus are subject to denaturation in certain conditions (Campbell and Reece, 2002). 1
Enzymes are granted the task of breaking bonds within the monomers of substrate, the molecule upon which the enzyme is acting. When the substrate and the enzyme bind, the substrate-enzyme complex is formed. The substrate binds to the enzyme’s active site, which is the part of the protein in which the enzyme fits. Scientists have introduced multiple models that attempt to illustrate exactly how the enzyme and substrate fit together (See figure 1). In the lock and key model, the substrate and enzyme fit together perfectly. In the induced fit model, however, the enzyme changes to fit the substrate, which secures the substrate-enzyme complex even further. Induced fit allows the enzyme to position the substrate so that its ability to catalyze is enhanced (Campbell and Reece, 2002). Once the enzyme has broken down or built up the reactant(s) and released the product(s), it bonds, unaffected, to another substrate (What are Enzymes?, 2010).
In order for an enzyme to work efficiently, it must reduce the activation energy, or the initial energy input to start the reaction and break or form the bonds between monomers. The quantity of activation energy needed to start a reaction is very great, which is the reason that reactions do not occur by themselves. The enzyme applies stress to certain chemical bonds of the molecules. The bending of such bonds decreases the amount of energy, which is usually thermal, needed to begin the reaction. Less input is required for the same reaction (See figure 2). A real life situation to describe this is pushing a boulder up a hill. The initial energy input is great and may require the use of machinery or many men. However, if the hill were to be diminished, the boulder would be able to get to its desired location with much less effort. In enzymes, a catalyzed reaction still has the same change in free energy or energy available for use (called ΔG) as an unanalyzed one, but with less free energy going into the reaction. Figure 2
When a reaction is endergonic, free energy is made available (as shown by –ΔG, see figure 2). An exergonic reaction requires an input of energy and in the end does not make any energy available.
Enzymes are very specific in the molecules with which they bind. In fact, they can even recognize very similar molecule, such as isomers, as not being their substrate “partner”. This specificity leads to many different types of enzymes all contributing to the chemical processes of life. A complex molecule is passed through a metabolic pathway consisting of many enzymes in order to get to the final product. Figure 2
Enzymes are naturally controlled so that only needed products are made. Allosteric enzymes, which are constructed from a few polypeptide chains, control the rates of reactions in metabolic pathways and when enzymes are active. Allosteric enzymes consist of inhibitors as well as activators. They bind to a specific site on an enzyme, called the allosteric site, and change the enzymes shape so that it is either active or inactive, depending on the allosteric...