CONCEPTUALIZACIÓN DE LA EDUCACIÓN INFANTIL

There are two major considerations with regards to the mechanism of enzyme action:

substrate specificity and catalytic power.

Chemical catalysts display only limited selectivity whereas enzymes show specificity for the substrates and also products. This ensures that the final product is not contaminated with by-products. In the enzyme catalysis the reaction takes place in a particular region that is designed to accommodate the specific participants involved in the reaction. The region, which is known as the active site, binds the substrate and then carries out the reaction. Enzymes with broad specificity have more flexible active site requirements and can therefore accept a wider

C H

NH ₂ COOH R

alpha carbon atom

Chapter 3. Enzymes

range of substrate molecules (Reiner, 1969; Blackburn, 1976; Bickerstaff, 1987; Copeland, 2000).

During any reaction there is a state which is called "transition state". In this state the susceptible substrate bonds are not completely broken and new bonds in the product are not completely formed. The state is energy dependent because it requires energy to make and break chemical bonds. This represents an energy barrier to successful reaction, and is the reason why the vast majority of reactions proceed extremely slowly in the absence of external help. By providing heat energy, high pressure or extreme pH to weaken bonds, or by the addition of catalysts, reactants can be helped toward the transition state. To reduce the energy barrier, enzymes catalysts are more effective than other catalysts to facilitate transition state formation and thereby increase the rate of reaction (Reiner, 1969; Blackburn, 1976;

Bickerstaff, 1987; Copeland, 2000).

The catalytic power of enzymes is due to the precise molecular interactions that occur at its active site. These interactions lower the energy barrier and make formation of the transition state easier. There are at least four types of interactions that can accomplish this effect, and they may operate singly or in combination.

x First, the active site in many enzymes provides a non-polar micro-environment. The removal of the substrate molecule from an aqueous polar solution into a non-polar phase may alter the conformation of the substrate towards the transition state. Also, a non-polar environment is useful for excluding water molecules, which may interfere in a reaction.

x Second, the precise alignment of substrate molecules in the active site presents the susceptible bonds at the correct angle so that a collision between reactants will result in the formation of a transition state.

x Third, the substrate molecule is normally held firmly in the active site by a number of non-covalent interactions, and small movements in the conformation of the enzyme molecule can be transmitted to active site causing a distortion of substrate structure, weakening the susceptible bond and reducing the amount of energy required to form a transition state.

x Lastly, the site amino acid residues contribute catalytic functional groups to participate directly in the reaction.

Chapter 3. Enzymes

The basic mechanism by which enzymes catalyze chemical reactions begins with the binding of the substrate (or substrates) to the active site on the enzyme. The binding of the substrate to the enzyme causes changes in the distribution of electrons in the chemical bonds of the substrate and ultimately causes the reactions that lead to the formation of products. The products are released from the enzyme surface to regenerate the enzyme for another reaction cycle (see Figure 3.2).

Figure 3.2: General mechanism of enzymes to catalyze chemical reactions.

The active site has a unique geometric shape that is complementary to the geometric shape of a substrate molecule, similar to the fit of puzzle pieces. This means that enzymes specifically react with only one or a very few similar compounds. There are two theories which describe the binding of enzymes and substrates. The first one is the lock and key theory, and the second one is the induced fit theory, which is a modification of the lock and key model.

3.2.1 Lock and Key Theory

The specific action of an enzyme with a single substrate can be explained using a Lock and Key analogy first postulated in 1894 by Emil Fischer (Meyer, 1995). In this analogy, the lock is the enzyme and the key is the substrate (see Figure 3.2). Only the correctly sized key (substrate) fits into the key hole (active site) of the lock (enzyme).

Smaller keys, larger keys, or incorrectly positioned teeth on keys (incorrectly shaped or sized substrate molecules) do not fit into the lock (enzyme). Only the correctly shaped key opens a particular lock.

Chapter 3. Enzymes

3.2.2 Induced Fit Theory

Not all experimental evidence can be adequately explained by using the so-called rigid enzyme model assumed by the lock and key theory. For this reason, a modification called the induced-fit theory has been proposed by Koshland (1958).

The induced-fit theory assumes that the substrate plays a role in determining the final shape of the enzyme and that the enzyme is partially flexible. In this theory substrate binding induces a change in enzyme conformation which cause enzyme and substrate fit together better and so that groups in the active site which are required for catalysis are properly positioned (see Figure 3.3). This theory can also explain why sometimes certain compounds can bind to the enzyme but do not react properly, because the enzyme has been distorted too much.

Figure 3.3: Mechanisms which describe the binding of enzymes and substrates.