A catalyst is a substance that enters into the process of a reaction. It might undergo temporary change in the molecular structure, but ultimately the catalyst molecule is restored to its original structure before the product has been formed. Catalysts generally reduce the Energy of Activation so that product might be formed at a lower temperature. (Fig 1)

In some cases, if it weren't for the catalyst, the reaction would occur so slowly that practical product formation would not be feasible. From a molecular point of view the catalyst provides a surface for the reactant molecules to position themselves with one another so that when they do collide they will do so much more effectively. There are two kinds of catalysts:

1. Heterogeneous catalysts

2. Homogeneous catalysts

Heterogeneous catalysts are sometimes called surface catalysts because they position the reactant molecules on their very surface. Many metals serve as heterogeneous catalysts in which the reactant molecules have an interface between themselves and the catalyst surface. In the reaction known as Hydrogenation, double bonds between carbons accept two hydrogen atoms and use the Pi electrons between the two carbons in order to attach these hydrogen atoms to the carbon atom. The di-atomic Hydrogen molecule attaches itself to the surface of a metal catalyst such as Platinum, Nickel, or Paladium. The double bonded organic molecule does the same. The single bond between the Hydrogen atoms is broken, and so is the Pi bond between the two carbons within the organic molecule broken. The Hydrogen atoms then form a single bond with its single electron and one of the two Pi electrons that previously constituted the Pi bond between the two carbon atoms. Once the Hydrogens have been attached the product molecule disengages from the surface only to have fresh reactant molecules take its place upon the surface of the metal. Heterogeneous catalysts are, as a rule, not as efficient as homogeneous catalysts.

Homogeneous catalysts are catalysts that form a uniform distribution between themselves and the reactant molecules. These catalysts are in a solution along with the reactant molecules. Because they are dispersed within a solution, the surface area of the catalyst is maximized, and usually, these types of catalysts tend to be more efficient in increasing product formation at a lower temperature. Examples of homogeneous catalysts would include any protic acid. A protic acid would be an acid that donates Hydrogen ions(protons). Sulfuric Acid, H₂SO₄, in water catalyze dehydrations of alcohols (ie:loss of water) to produce double bonded compounds called alkenes. Phosphoric Acid, H₃PO₄, can catalyze the formation of organic esters by the combining of an alcohol and a carboxylic acid molecules together splitting out a water molecule from between the two molecules.

Aprotic acids also serve as homogeneous catalysts. An aprotic acid is an acid that accepts electrons. It is called a Lewis acid. Aprotic acids such as AlCl₃ can catalyze the substitution reactions in which a hydrogen atom on a Benzene ring is replaced by a hydrocarbon group such as a methyl group,-CH₃. This same aprotic acid is also useful in catalyzing the Chlorination of an alkene to form a di-chloro alkane.

Probably the most amazing and incredably efficient homogeneous catalysts are the proteins that make up the enzymes. Enzymes are biochemical catalysts with an amazing efficiency far outstripping the other homogeneous and heterogeneous catalysts. One enzyme may be responsible for catalyzing the formation of 1000-10,000 or up to as many as one million product molecules. Reaction rates can increase by 10⁸ to 10¹¹ times faster than the uncatalyzed reaction. In addition, enzymes have an uncanny specificity in that it is rare for a specific enzyme to catalyze more than one specific reaction.

Enzymes are made up of a protein part called the apoenzyme and a non-protein part called the Prosthetic group. There is a model that seeks to explain how enzymes function with such a high degree of efficiency and specificity. The earliest model was called the "lock and key" model of enzyme activity. In this model the apoenzyme has one or more cavities on the surface of this macro-molecular system which can bind a reactant molecule called a substrate to the cavity. This cavity or indentation is called the "active site". According to this model, once the substrate molecule has been positioned with the help of the prostetic group and perhaps a Co-factor metal ion, then the bond breaking and bond formation can begin. Once the product has been formed, the product disengages from the active site in order for the process to be repeated. According to this model, the shape of the substrate molecule must be compatible to the active site like a key is compatible with a specific lock. This would explain the unusual high specificity of enzymes capable of only catalyzing one reaction. If the wrong substrate tries to fit into the active site, it does not fit (wrong key) and so the enzyme does not catalyze its conversion to some other product. If the active site of the enzyme is stretched or compressed then even the real substrate molecule will not fit the distorted active site. The problem with this model is that it does not explain why some bogus substrate molecules can "fool" the enzyme into thinking the bogus is the real thing. The bogus molecule fits into the active site even though it is not the exact same size as the real thing. Once the bogus molecule is in place it just sits there and blocks the active site. This inhibits the enzyme from doing its job and is referred to as active site blocking.

The Induced Fit model is a modification of the lock and key model. Instead of having a ridged substrate molecule that will only fit into a specific active site, we have an active site that can stretch to accept a range of molecular shapes for the substrate analogous to how gloves stretch to fit more than one size hand. This model explains more adequately how active site blockers can block the active site and inhibit the enzyme molecule.

There are really two types of inhibitors:

1. Competitive Inhibitors- These are molecules that actually block the active sites and "compete" for the locked position of the active site.

2. Non-competitive Inhibitors- These are molecules that attach themselves to other parts of the apoenzyme away from the active site. However, when the inhibitor bonds to the enzyme molecule, it alters the shape, and therefore, the shape of the active site is altered so that the active site will no longer accept a legitimate substrate molecule.

There are three factors that will alter the activity of an enzyme.

1. Temperature- Every enzyme has a temperature range in which the enzyme can function within. Usually on the low end of that range the enzyme has low activity.Near the middle part of the range, the activity of the enzyme increases to a maximum called the optimum temperature. Outside that range, the enzyme ceases to function as a catalyst. The temperature range of most of the hundreds of enzymes found in the human organism is 10-50 degrees Celsius with the optimum temperature being 37 C. If a human becomes hypothermic with a abnormally low body temperature, then enzymes cease to function and biochemical reactions within the cells begin to shut down. If the human becomes hyperthermic with an abnormally high body temperature, then the enzymes activity drops rapidly, and the reactions of the cells begin to shut down once again. If the body temperature gets too far away from the optimum temperature, then the body may cease to function. At what temperature this occurs really depends upon the individual. Every individual has there own tolerances of temperature variation. What we do is usually deal with a statistical average which is really non-existent, but takes in most individuals. When the body experiences extremes in temperature, then the body usually begins shutting down non-essential systems to life support and the individual goes into a coma. Coming out of a coma usually depends but not always on correcting the temperature extreme. High temperatures usually inhibit enzyme activity by denaturing the protein enzyme. The temperature will cause the protein shape to be so distorted that the active site is distorted to the point that the substarte is no longer accepted.

2. Acidity- Every enzyme functions within a pH range with an optimal pH within that range. Enzymes are extremely sensitive to pH changes so their ranges tend to be rather narrow. For example, the enzymes that catalyze blood chemistry must function within a tiny range of 7.35-7.45 in plasma. If the pH drops outside that range on the low end then a condition known as acidosis occurs and enzymes cease to function and the blood chemistry shuts down. On the other hand if the pH goes outside that 0.1 pH range on the high side, we have a condition known as alkalosis. If this should happen the structure and shape of the enzyme is altered so drastically that the active sites no longer can accept the substarte molecules because of their distortion. Changing the pH disrupts intramolecular binding forces that keep the enzyme in a specific shape. Disrupting those forces will unravel the protein and render the active site distorted.

3. Active site blocking- We have already discussed how competitive inhibitors can affect the enzyme activity.

Toxicolgy is the study of toxic agents. Toxic agents is a little disceptive. Most people think of something that is toxic as something that will cause certain death. This is not always the case. Toxicity depends on a number of factors. The body weight of the organism has to be considered. The tolerance level of the toxin in the organism is a factor. The dosage of toxin ingested and the time interval of the ingestion all have to be considered as important to gauge just how toxic a toxin is. To use a ridiculous hyperbole, one can die of too much water due to kidney shutdown, but we would not think of calling water a toxin. Depending on the dosage and body weight some toxins only nausiate whereas others cause vomiting, convulsions, coma, and, yes, even death.

Some toxins such as heavy metals like Mercury,Lead, and other transition metals will denature enzymes causing them to be precipitated along with the metal. In many of these toxins the antidote is to ingest something that will precipitae the metals like , in some cases, drinking milk where the protein in the milk casein will precipitate the metals and allow them to be vomited or passed out of the body in some way. Other toxins function as active site blockers inhibiting an enzyme from catalyzing a necessary biochemical reaction. For example, Cyanide is a toxin which will block the active sites of some enzymes called cytochomases that are responsible for cellular respiration. When these enzymes are blocked by the cyanide then cells die of asphixiation. Many insecticides and pesticides work on the principle of blocking the active sites of enzymes within an insect or plant that affect the growth of the organism or the central nervous system of the organism shutting it down. The problem with many such pesticides and herbacides is that they can easily be harmful to humans if ingested through the mouth, nose or in some cases the pores of the skin. There are a group of toxins called neurotoxins that affect the neuromuscular system. These substance block the active site of an important enzyme of the neuromuscular system called acetylcholine esterase. This enzyme is responsible for hydrolyzing acetyl choline as an ester after it was produced in order to initiate muscular contraction. The problem is that if the resulting acetylcholine molecule is not converted back to choline with the aid of the acetylcholine esterase, then muscles cannot return to a relaxed state which could result in paralysis of the heart beat, respiration and other processes controlled involuntarily by the neuromuscular system. Massive doses of neurotoxins will result in the respiratory system, the nuromuscular system and the cardiovascular systems being paralyzed and death occuring rapidly.

Lest I leave you with the impression that all enzyme inhibition results in bad things happening, let me cite successes in chemotheraputic treatment of cancer. It was shown earily in this research that an organic base derivative 5-fluro uracil was responsible for arresting the growth of cancer cells by inhibiting an enzyme responsible for synthesizing the DNA of cancer cells. There are many problems with such agents in that in far too many cases the same agent that kills cancer cells will indiscriminately kill normal cells as well. However, research is proceeding in developing support medicines that will control the negative side effects in using such chemotherapeutic agents such as nausia, loss of hair,skin lessons, and loss of apetite. In addition, new drugs are being developed that are more discriminating on killing the cancer cells leaving the normal cells intact. In addition, certain muscle relaxants such as succinylcholine act as a substitute for acetylcholine in binding to muscle receptor sites. Succinylcholine causes muscle relaxation instead of contraction and since the acetylcholine esterase can't hydrolyze the Succinylcholine as efficiently as acetylcholine, the muscle remains in a relaxed state longer.

R. H. Logan, Instructor of Chemistry, Dallas County Community College District, North Lake College.

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All textual content copyrighted (c) 1997
R.H. Logan, Instructor of Chemistry, DCCCD
All Rights reserved

Revised: 3/9/97

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