The most efficient and amazing catalysts of all are the biological molecules known as enzymes. One of these, nitrogenase, is found in algae, bacteria, and microorganisms associated with leguminous plants such as soybeans. Nitrogenase can fix nitrogen from the atmosphereA unit of pressure equal to 101.325 kPa or 760 mmHg; abbreviated atm. Also, the mixture of gases surrounding the earth. at room temperatureA physical property that indicates whether one object can transfer thermal energy to another object. (instead of at 500°C as in the Haber process). It does so efficiently enough that only a few kilograms of enzyme are necessary to fix about 44 × 1012 g of nitrogen worldwide each year. Other enzymes carry out other reactions with comparable efficiencies.
All enzymes are large proteinA biological polymer of amino acids joined by peptide bonds. molecules whose molar masses exceed 20 000 g mol–1. They are condensation polymers of amino acids, and a typical section of a long protein chain may be represented as
The alternate red and black segments each represent a different amino acid, and the symbols R1, R2, R3, etc., stand for different organicRefers to the branch of chemistry that studies compounds containing carbon, usually in combination with hydrogen and other elements such as O, N, S, and P. Certain small ions and compounds containing carbon (such as carbonate ions and carbon dioxide) are not considered to be organic, but rather are classed as inorganic. groups called side chains. In the amino acid glycine, for instance, the side chain is a hydrogen atomThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume., while for glutamic acid it is the group —CH2—CH2—COOH. A typical enzyme molecule contains as many as 20 different kinds of amino acids, while the chain may be as long as 200 or 300 amino acid units. (More details on the amino acids, their side chains, and the primary, secondary, and higher order structures of proteins are given in their respective sections.)
The polymer chain of amino acid units in an enzyme is usually wrapped around itself in a complex fashion to form an overall shape which is approximately spherical. The conformationIn an ion or molecule, one of many spatial arrangements of atoms that differ by rotation about single bonds, but that do not differ in which atoms are attached to each other. of the protein chain in the enzyme trypsin has been determined experimentally and is shown in Fig. 1. The chain appears quite random, almost as though it were a ball of tangled string or yarn, but there are certain molecular features in the region shown in color in the diagram which are essential to its operation as a catalyst. Accordingly, this region of the molecular surface is called the active site.
An important feature of the catalytic behavior of enzymes is their specificity. Most enzymes are able to catalyze only one particular chemical reactionA process in which one or more substances, the reactant or reactants, change into one or more different substances, the products; chemical change involves rearrangement, combination, or separation of atoms. Also called chemical change. or at most only a few closely related reactions. Some interesting examples of specificity involve the enzymes which, as part of the process of digestion, catalyze the breakdown of other proteins into their constituent amino acids in the stomach and intestines. These enzymes all catalyze hydrolysisAny reaction in which water (hydro) is split into two parts (lysis). Examples include the reaction of an anion with water to form the conjugate acid and hydroxide ion and hydrolysis of an ester or amide, in which the H from water bonds to form an alcohol or amine and the OH bonds to a carbonyl carbon to form a carboxylic acid. of the C—N bond in the amino acid chain according to the following equation:
When all the C—N bonds in a protein are broken, it is transformed entirely into amino acids. Amino acids can penetrate the walls of the intestine and be utilized by the body to build up other protein chains.
Hydrolysis of this type of C—N bond is also catalyzed by acids and bases. Thus, for instance, a standard laboratory method for breaking up a protein into its constituent amino acids is to heatEnergy transferred as a result of a temperature difference; a form of energy stored in the movement of atomic-sized particles. it overnight at 100°C in 1 M HCl. Despite the higher temperature, this acid-catalyzed hydrolysis is much slower than one catalyzed by an enzyme, and all the C—N bonds are broken, no matterAnything that occupies space and has mass; contrasted with energy. what amino acids are involved. When an enzyme is used, by contrast, only certain C—N bonds are hydrolyzed. The enzyme carboxy-peptidase will only catalyze the hydrolysis of a C—N bond at the end of an amino acid chain. On the other hand, trypsin will only operate if one of the amino acids involved in the bond is lysine or arginine.
Enzymes usually behave very simply from the point of view of chemical kinetics. The rate lawAn equation which describes the rate of a reaction as a function of the rate constant and the concentrations of reactants (and any other substances that affect the rate, such as products or catalysts); also called rate equation. is usually found to be first order with respect to the enzyme concentrationA measure of the ratio of the quantity of a substance to the quantity of solvent, solution, or ore. Also, the process of making something more concentrated. and also first order with respect to the substrateThe molecule or ion that is a reactant in an enzyme-catalyzed reaction. (the substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. on which the enzyme acts). In other words
Rate = k(cenzyme)(csubstrate)
Thus the activated complexIn the mechanism of a reaction, a species that lies at an energy peak and that can change either into products or into reactants; also called a transition state. consists of a combination of one enzyme molecule and one substrate molecule. Since the enzyme molecule is usually much larger than the substrate molecule, we can think of the substrate molecule as being held down on the surface of the enzyme and enzyme catalysisThe increase in rate of a reaction due to the presence of a substance that undergoes no net change during the reaction. as being related to heterogeneous catalysis.
Using multiple techniques, such as X-rayA high-energy form of electromagnetic radiation that has sufficient energy to ionize inner electrons from an atom. crystallography to determine three-dimensional structures of enzymes, as well as experimental kinetics, biochemists can understand the finer details of how an enzyme is able to operate so efficiently as a catalyst. Among the best understood is the enzyme trypsin, which, as we have just mentioned, catalyzes the hydrolysis of C—N bonds attached only to the amino acids lysine and arginine. A somewhat oversimplified and schematic account of the operation of this enzyme is given in Fig. 2.
The active site on the surface of the enzyme, shown in gray, contains an opening or pocket which is just large enough to accommodate the long side chains of lysine or arginine, and which also contains a negative ionic charge deep inside it. Side chains belonging to other amino acids are the wrong size or shape or are not positively charged, and so they cannot be held in this pocket, but an arginine or lysine side chain can be. Once the side chain is held by the pocket, there is an acidic hydrogen atom on the enzyme in exactly the right position to protonate the nitrogen atom in a C—N bond in the protein chain. The enzyme thus functions as an acid catalyst for the hydrolysis of the C—N bond. It is, of course, much more efficient than a nonen-zymic catalyst such as HCI. Partly this is because the proton is in exactly the right place to donate once the side chain has entered the pocket. Partly, also, it is because there is considerable lowering in the activation energyThe energy barrier over which a reaction must progress in order for reactants to form products; the minimum energy that reactants must have if they are to be converted to products. due to the ionic attraction between the enzyme and substrate.