Enzymes

Any reactions in order to start and to be completed require not only the presence of the specific reactants, but also a minimum quantity of energy: if the energy of the reactants is not sufficient to activate all the mechanisms that complete the reaction, the mixture ofreactants stays inert. The minimum energy required, called activation energy, strictly depends on the reaction: for some reactions, this value is low and hence they occur spontaneously, whereas other reactions require a high activation energy and cannot occur without an external support, viz. supplying more energy to the reactants (mainly increasing the temperature) or adding one or more catalysts that reduce the activation energy. Catalysis is a very important factor in ecological processes because most of the reactions that involve biological organisms need an high activation energy and hence they could not occur otherwise. The most important class of catalysts for ecological processes is enzymes: they are proteins produced by organisms themselves.

The function of a specific enzyme is determined by its structure: in the complex three-dimensional (3-D) structure ofthese proteins exists a small part (called active site) that is able to bind the reactants (called substrates) and hence speed up the reaction.

Enzymatic reactions can be described as a two-step reaction (Figure 1): in the first step, enzyme and substrate bind themselves to create the complex enzyme substrate, and in the second step the complex enzyme substrate reacts to generate products and free enzyme.

The binding provides better conditions (physical and/ or chemical) to activate the reaction: the activation energy of both reactions is much lower than the energy required by the primitive reaction (Figure 2); hence globally the reaction is fostered.

It is important to note that globally (i.e., after an infinite time), the amount of reactants that are transformed in products does not depend on the presence of enzyme: indeed, enzymes do not change the equilibrium of reactions but they simply permit to reduce the time needed to reach the equilibrium.

The enzymes are usually built to foster a specific reaction; hence, it is necessary that the specificity of the

Products

Substrate Substrate

Enzyme Enzyme-substrate complex Enzyme

Figure 1 Schema of enzymatic reactions: in the first step substrate binds itself to the active site of the enzyme to form the enzymesubstrate complex, then substrate is transformed into products and enzyme is available to bind a new molecule of substrate.

Figure 2 Diagram of the energy during a reaction without enzymes (dashed line) and during the same reaction with enzymes (solid line). The activation energy for catalyzed reaction (AGcat) is a lot lower than the activation energy needed without enzymes (AG). Also evidenced is the metastable state of the enzyme-substrate complex (ES).

Figure 2 Diagram of the energy during a reaction without enzymes (dashed line) and during the same reaction with enzymes (solid line). The activation energy for catalyzed reaction (AGcat) is a lot lower than the activation energy needed without enzymes (AG). Also evidenced is the metastable state of the enzyme-substrate complex (ES).

active site is very high: this is achieved combining several physical and chemical properties of enzyme and substrate. Nevertheless the link between active site and substrate is not unique: different substrates could compete for the same active site in order to achieve different reactions (e.g., CO2 and O2 compete to bind to ribulose-1,5-bispho-sphate carboxylase/oxygenase - RuBisCO - in order to fix carbon or to oxidize the sugar via photorespiration); or other substances, called inhibitors, can be bound to the active site without the activation of any reaction. The selective capability of the enzyme is expressed by the specificity factor, defined as the ratio of the consumption rates of the two competing substances, assuming that they have equal concentration. When an inhibitor binds itself to an enzyme, it temporarily prevents any other bonds, that is, the formation of the enzyme-substrate complex: the global consequence is a reduction of the reaction rate. Enzymatic reaction can also be limited by uncompetitive inhibitors; these substances are bound by the enzyme in another place of the structure and prevent the activation of the reaction even if the enzyme-substrate complex is still possible.

As any other catalysts, enzymes are not consumed in the reaction; hence, after the completion of the reaction, the bond between enzyme and substrate is broken and the active site is available to bind another molecule of substrate: the number of molecules of substrates that can be bound and converted to the product per catalytic site per unit time is called turnover number.

10 Ways To Fight Off Cancer

10 Ways To Fight Off Cancer

Learning About 10 Ways Fight Off Cancer Can Have Amazing Benefits For Your Life The Best Tips On How To Keep This Killer At Bay Discovering that you or a loved one has cancer can be utterly terrifying. All the same, once you comprehend the causes of cancer and learn how to reverse those causes, you or your loved one may have more than a fighting chance of beating out cancer.

Get My Free Ebook


Post a comment