Enzymes are key protein molecules in living systems. Once synthesized, they are not usually converted to other kinds of molecules, and so are the substances taken in as fuel for digestive and respiratory processes (such as sugar, fat, and molecular oxygen). This is because enzymes are catalysts, which means they can participate in chemical reactions without themselves being changed, just like the moderator of a public debate who ideally moves the participants and the audience toward a conclusion by dictating the terms of the argument while not adding any unique information.

More than 2,000 enzymes have been identified, and each enzyme is involved with one specific chemical reaction. Enzymes are therefore substrate-specific. They are grouped into a half-dozen classes on the basis of the kinds of reactions they take part in.

Enzyme Basics
Enzymes permit a vast number of reactions to take place in the body under conditions of homeostasis, or overall biochemical balance. For example, many enzymes function best at a pH (acidity) level close to the pH the body normally maintains, which is in the range of 7 (that is, neither alkaline nor acidic). Other enzymes function best at low pH (high acidity) because of the demands of their environment; for example, the inside of the stomach, where some digestive enzymes operate, is highly acidic.

Enzymes take part in processes ranging from blood clotting to DNA synthesis to digestion. Some are found only within cells and participate in processes involving small molecules, such as glycolysis; others are secreted directly into the gut and act on bulk matter such as swallowed food.

Because enzymes are proteins with fairly high molecular masses, they each have a distinct three-dimensional shape. This determines the specific molecules on which they act. In addition to being pH-dependent, the shape of most enzymes is temperature-dependent, meaning that they function best in a fairly narrow temperature range.

How Enzymes Work
Most enzymes work by lowering the activation energy of a chemical reaction. Sometimes, their shape brings the reactants physically close together in the style, perhaps, of a sports-team coach or work-group manager intent on getting a task done more quickly. It is believed that when enzymes bind to a reactant, their shape changes in a way that destabilizes the reactant and makes it more susceptible to whatever chemical changes the reaction involves.

Reactions that can proceed without the input of energy are called exothermic reactions. In these reactions, the products, or the chemical(s) formed during the reaction, have a lower energy level than the chemicals that serve as the reaction's ingredients. In this way, molecules, like water, "seek" their own (energy) level; atoms "prefer" to be in arrangements with lower total energy, just like water flows downhill to the lowest available physical point. Putting all of this together, it is clear that exothermic reactions always proceed naturally.

However, the fact that a reaction will occur even without input says nothing about the rate at which it will happen. If a substance taken into the body will naturally change into two derivative substances that can serve as direct sources of cellular energy, this does little good if the reaction naturally takes hours or days to complete. Also, even when the total energy of products is higher than that of the reactants, the energy path is not a smooth downhill slope on a graph; instead, the products must attain a higher level of energy than that with which they began so that they can "get over the hump" and the reaction may proceed. This initial investment of energy into the reactants that pays off in the form of products is the aforementioned energy of activation, or Ea.

Types of Enzymes
The human body includes six major groups, or classes, of enzymes.

Oxidoreductases enhance the rate of oxidation and reduction reactions. In these reactions, also called redox reactions, one of the reactants gives up a pair of electrons that another reactant gains. The electron-pair donor is said to be oxidized and acts as a reducing agent, while the electron-pair recipient is reduced is called the oxidizing agent. A more straightforward way of putting this is that in these kinds of reactions, oxygen atoms, hydrogen atoms or both are moved. Examples include cytochrome oxidase and lactate dehydrogenase.

Transferases speed along the transfer of groups of atoms, such as methyl, acetyl or amino groups, from one molecule to another molecule. Acetate kinase and alanine deaminase are examples of transferases.

Hydrolases accelerate hydrolysis reactions. Hydrolysis reactions use water to split a bond in a molecule to create two daughter products, usually by affixing the -OH (hydroxyl group) from the water to one of the products and a single -H (hydrogen atom) to the other. In the meantime, a new molecule is formed from the atom.

Lyase, physiologically, is any member of a class of enzymes that catalyze the addition or removal of elements of water (hydrogen, oxygen), ammonia (nitrogen, hydrogen) or carbon dioxide (carbon, oxygen) on a double bond. For example, a decarboxylase removes carbon dioxide from an amino acid, while a dehydratase removes water.

Isomerases are enzymes that catalyze the formation of substrate isomers. In other words, they promote the transfer of specific functional groups within the molecule without adding or removing atoms from the substrate. This conversion can be simply expressed as A→B, where A and B are isomers.

Ligases are enzymes that catalyze the transfer of O antigens (a polymer of repeating oligosaccharides) from the Und-PP-linked carrier to the lipid A core at the last step of LPS biosynthesis.

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I'm a writer and biologist living in New York.