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enzyme
Substance that acts as a catalyst in living organisms, regulating the rate at
which life's chemical reactions proceed without being altered in the process.
Enzymes reduce the activation energy needed to start these reactions; without
them, most such reactions would not take place at a useful rate. Because enzymes
are not consumed, only tiny amounts of them are needed. Enzymes catalyze all
aspects of cell metabolism, including the digestion of food, in which large
nutrient molecules (including proteins, carbohydrates, and fats) are broken down
into smaller molecules; the conservation and transformation of chemical energy;
and the construction of cellular materials and components. Almost all enzymes
are proteins; many depend on a nonprotein cofactor, either a loosely associated
organic compound (e.g., a vitamin; see coenzyme) or a tightly bound metal ion
(e.g., iron, zinc) or organic (often metal-containing) group. The
enzyme-cofactor combination provides an active configuration, usually including
an active site into which the substance (substrate) involved in the reaction can
fit. Many enzymes are specific to one substrate. If a competing molecule blocks
the active site or changes its shape, the enzyme's activity is inhibited. If the
enzyme's configuration is destroyed (see denaturation), its activity is lost.
Enzymes are classified by the type of reaction they catalyze: (1)
oxidation-reduction, (2) transfer of a chemical group, (3) hydrolysis, (4)
removal or addition of a chemical group, (5) isomerization (see isomer;
isomerism), and (6) binding together of substrate units (polymerization). Most
enzyme names end in -ase. Enzymes are chiral catalysts, producing mostly or only
one of the possible stereoisomeric products (see optical activity). The
fermentation of wine, leavening of bread, curdling of milk into cheese, and
brewing of beer are all enzymatic reactions. The uses of enzymes in medicine
include killing disease-causing microorganisms, promoting wound healing, and
diagnosing certain diseases.
Enzymes are biological catalysts, or chemicals that speed up the rate of
reaction between substances without themselves being consumed in the reaction.
As such, they are vital to such bodily functions as digestion, and they make
possible processes that normally could not occur except at temperatures so high
they would threaten the well-being of the body. A type of protein, enzymes
sometimes work in tandem with non-proteins called coenzymes. Among the processes
in which enzymes play a vital role is fermentation, which takes place in the
production of alcohol or the baking of bread and also plays a part in numerous
other natural phenomena, such as the purification of wastewater.
How It Works
Amino Acids, Proteins, and Biochemistry
Amino acids are organic compounds made of carbon, hydrogen, oxygen, nitrogen,
and (in some cases) sulfur bonded in characteristic formations. Strings of 50 or
more amino acids are known as proteins, large molecules that serve the functions
of promoting normal growth, repairing damaged tissue, contributing to the body's
immune system, and making enzymes. The latter are a type of protein that
functions as a catalyst, a substance that speeds up a chemical reaction without
participating in it. Catalysts, of which enzymes in the bodies of plants and
animals are a good example, thus are not consumed in the reaction.
Catalysts
In a chemical reaction, substances known as reactants interact with one another
to create new substances, called products. Energy is an important component in
the chemical reaction, because a certain threshold, termed the activation
energy, must be crossed before a reaction can occur. To increase the rate at
which a reaction takes place and to hasten the crossing of the activation energy
threshold, it is necessary to do one of three things.
The first two options are to increase either the concentration of reactants or
the temperature at which the reaction takes place. It is not always feasible or
desirable, however, to do either of these things. Many of the processes that
take place in the human body, for instance, normally would require high
temperatures—temperatures, in fact, that are too high to sustain human life.
Imagine what would happen if the only way we had of digesting starch was to heat
it to the boiling point inside our stomachs! Fortunately, there is a third
option: the introduction of a catalyst, a substance that speeds up a reaction
without participating in it either as a reactant or as a product. Catalysts thus
are not consumed in the reaction. Enzymes, which facilitate the necessary
reactions in our bodies without raising temperatures or increasing the
concentrations of substances, are a prime example of a chemical catalyst.
The Discovery of Catalysis
Long before chemists recognized the existence of catalysts, ordinary people had
been using the chemical process known as catalysis for numerous purposes: making
soap, fermenting wine to create vinegar, or leavening bread, for instance. Early
in the nineteenth century, chemists began to take note of this phenomenon. In
1812 the Russian chemist Gottlieb Kirchhoff (1764-1833) was studying the
conversion of starches to sugar in the presence of strong acids when he noticed
something interesting.
When a suspension of starch (that is, particles of starch suspended in water)
was boiled, Kirchhoff observed, no change occurred in the starch. When he added
a few drops of concentrated acid before boiling the suspension, however, he
obtained a very different result. This time, the starch broke down to form
glucose, a simple sugar (see Carbohydrates), whereas the acid—which clearly had
facilitated the reaction—underwent no change. In 1835 the Swedish chemist Jöns
Berzelius (1779-1848) provided a name to the process Kirchhoff had observed:
catalysis, derived from the Greek words kata ("down") and lyein ("loosen"). Just
two years earlier, in 1833, the French physiologist Anselme Payen (1795-1871)
had isolated a material from malt that accelerated the conversion of starch to
sugar, for instance, in the brewing of beer.
The renowned French chemist Louis Pasteur (1822-1895), who was right about so
many things, called these catalysts ferments and pronounced them separate
organisms. In 1897, however, the German biochemist Eduard Buchner (1860-1917)
isolated the catalysts that bring about the fermentation of alcohol and
determined that they were chemical substances, not organisms. By that time, the
German physiologist Willy Kahne had suggested the name enzyme for these
catalysts in living systems.
Substrates and Active Sites
Each type of enzyme is geared to interact chemically with only one particular
substance or type of substance, termed a substrate. The two parts fit together,
according to a widely accepted theory introduced in the 1890s by the German
chemist Emil Fischer (1852-1919), as a key fits into a lock. Each type of enzyme
has a specific three-dimensional shape that enables it to fit with the
substrate, which has a complementary shape.
The link between enzymes and substrates is so strong that enzymes often are
named after the substrate involved, simply by adding ase to the name of the
substrate. For example, lactase is the enzyme that catalyzes the digestion of
lactose, or milk sugar, and urease catalyzes the chemical breakdown of urea, a
substance in urine. Enzymes bind their reactants or substrates at special folds
and clefts, named active sites, in the structure of the substrate. Because
numerous interactions are required in their work of catalysis, enzymes must have
many active sites, and therefore they are very large, having atomic mass figures
as high as one million amu. (An atomic mass unit, or amu, is approximately equal
to the mass of a proton, a positively charged particle in the nucleus of an
atom.)
Suppose a substrate molecule, such as a starch, needs to be broken apart for the
purposes of digestion in a living body. The energy needed to break apart the
substrate is quite large, larger than is available in the body. An enzyme with
the correct molecular shape arrives on the scene and attaches itself to the
substrate molecule, forming a chemical bond within it. The formation of these
bonds causes the breaking apart of other bonds within the substrate molecule,
after which the enzyme, its work finished, moves on to another uncatalyzed
substrate molecule.
Coenzymes
All enzymes belong to the protein family, but many of them are unable to
participate in a catalytic reaction until they link with a non protein component
called a coenzyme. This can be a medium-size molecule called a prosthetic group,
or it can be a metal ion (an atom with a net electric charge), in which case it
is known as a cofactor. Quite often, though, coenzymes are composed wholly or
partly of vitamins. Although some enzymes are attached very tightly to their
coenzymes, others can be parted easily; in either case, the parting almost
always deactivates both partners.
The first coenzyme was discovered by the English biochemist Sir Arthur Harden
(1865-1940) around the turn of the nineteenth century. Inspired by Buchner, who
in 1897 had detected an active enzyme in yeast juice that he had named zymase,
Harden used an extract of yeast in most of his studies. He soon discovered that
even after boiling, which presumably destroyed the enzymes in yeast, such
deactivated yeast could be reactivated. This finding led Harden to the
realization that a yeast enzyme apparently consists of two parts: a large,
molecular portion that could not survive boiling and was almost certainly a
protein and a smaller portion that had survived and was probably not a protein.
Harden, who later shared the 1929 Nobel Prize in chemistry for this research,
termed the non protein a coferment, but others began calling it a coenzyme.
Real-Life Applications
The Body, Food, and Digestion
Enzymes enable the many chemical reactions that are taking place at any second
inside the body of a plant or animal. One example of an enzyme is cytochrome,
which aids the respiratory system by catalyzing the combination of oxygen with
hydrogen within the cells. Other enzymes facilitate the conversion of food to
energy and make possible a variety of other necessary biological functions.
Enzymes in the human body fulfill one of three basic functions. The largest of
all enzyme types, sometimes called metabolic enzymes, assist in a wide range of
basic bodily processes, from breathing to thinking. Some such enzymes are
devoted to maintaining the immune system, which protects us against disease, and
others are involved in controlling the effects of toxins, such as tobacco smoke,
converting them to forms that the body can expel more easily.
A second category of enzyme is in the diet and consists of enzymes in raw foods
that aid in the process of digesting those foods. They include proteases, which
implement the digestion of protein; lipases, which help in digesting lipids or
fats; and amylases, which make it possible to digest carbohydrates. Such enzymes
set in motion the digestive process even when food is still in the mouth. As
these enzymes move with the food into the upper portion of the stomach, they
continue to assist with digestion.
The third group of enzymes also is involved in digestion, but these enzymes are
already in the body. The digestive glands secrete juices containing enzymes that
break down nutrients chemically into smaller molecules that are more easily
absorbed by the body. Amylase in the saliva begins the process of breaking down
complex carbohydrates into simple sugars. While food is still in the mouth, the
stomach begins producing pepsin, which, like protease, helps digest protein.
Later, when food enters the small intestine, the pancreas secretes pancreatic
juice—which contains three enzymes that break down carbohydrates, fats, and
proteins—into the duodenum, which is part of the small intestine. Enzymes from
food wind up among the nutrients circulated to the body through plasma, a watery
liquid in which red blood cells are suspended. These enzymes in the blood assist
the body in everything from growth to protection against infection.
One digestive enzyme that should be in the body, but is not always present, is
lactase. As we noted earlier, lactase works on lactose, the principal
carbohydrate in milk, to implement its digestion. If a person lacks this enzyme,
consuming dairy products may cause diarrhea, bloating, and cramping. Such a
person is said to be "lactose intolerant," and if he or she is to consume dairy
products at all, they must be in forms that contain lactase. For this reason,
Lactaid milk is sold in the specialty dairy section of major supermarkets, while
many health-food stores sell lactaid tablets.
Fermentation
Fermentation, in its broadest sense, is a process involving enzymes in which a
compound rich in energy is broken down into simpler substances. It also is
sometimes identified as a process in which large organic molecules (those
containing hydrogen and carbon) are broken down into simpler molecules as the
result of the action of microorganisms working anaerobically, or in the absence
of oxygen. The most familiar type of fermentation is the conversion of sugars
and starches to alcohol by enzymes in yeast. To distinguish this reaction from
other kinds of fermentation, the process is sometimes termed alcoholic or
ethanolic fermentation.
At some point in human prehistory, humans discovered that foods spoil, or go
bad. Yet at the dawn of history—that is, in ancient Sumer and Egypt—people found
that sometimes the "spoilage" (that is, fermentation) of products could have
beneficial results. Hence the fermentation of fruit juices, for example,
resulted in the formation of primitive forms of wine. Over the centuries that
followed, people learned how to make both alcoholic beverages and bread through
the controlled use of fermentation.
Alcoholic Beverages
In fermentation, starch is converted to simple sugars, such as sucrose and
glucose, and through a complex sequence of some 12 reactions, these sugars then
are converted to ethyl alcohol (the kind of alcohol that can be consumed, as
opposed to methyl alcohol and other toxic forms) and carbon dioxide. Numerous
enzymes are needed to carry out this sequence of reactions, the most important
being zymase, which is found in yeast cells. These enzymes are sensitive to
environmental conditions, such that when the concentration of alcohol reaches
about 14%, they are deactivated. For this reason, no fermentation product (such
as wine) can have an alcoholic concentration of more than about 14%. Stronger
alcoholic beverages, such as whisky, are the result of another process,
distillation.
The alcoholic beverages that can be produced by fermentation vary widely,
depending primarily on two factors: the plant that is fermented and the enzymes
used for fermentation. Depending on the materials available to them, various
peoples have used grapes, berries, corn, rice, wheat, honey, potatoes, barley,
hops, cactus juice, cassava roots, and other plant materials for fermentation to
produce wines, beers, and other fermented drinks. The natural product used in
making the beverage usually determines the name of the synthetic product. Thus,
for instance, wine made with rice—a time-honored tradition in Japan—is known as
sake, while a fermented beverage made from barley, hops, or malt sugar has a
name very familiar to Americans: beer. Grapes make wine, but "wine" made from
honey is known as mead.
Other Foods
Of course, ethyl alcohol is not the only useful product of fermentation or even
of fermentation using yeast; so, too, are baked goods, such as bread. The carbon
dioxide generated during fermentation is an important component of such items.
When the batter for bread is mixed, a small amount of sugar and yeast is added.
The bread then rises, which is more than just a figure of speech: it actually
puffs up as a result of the fermentation of the sugar by enzymes in the yeast,
which brings about the formation of carbon dioxide gas. The carbon dioxide gives
the batter bulkiness and texture that would be lacking without the fermentation
process. Another food-related application of fermentation is the production of
one processed type of food from a raw, natural variety. The conversion of raw
olives to the olives sold in stores, of cucumbers to pickles, and of cabbage to
sauerkraut utilizes a particular bacterium that assists in a type of
fermentation.
Industrial Applications
There is even ongoing research into the creation of edible products from the
fermentation of petroleum. While this may seem a bit far-fetched, it is less
difficult to comprehend powering cars with an environmentally friendly product
of fermentation known as gasohol. Gasohol first started to make headlines in the
1970s, when an oil embargo and resulting increases in gas prices, combined with
growing environmental concerns, raised the need for a type of fuel that would
use less petroleum. A mixture of about 90% gasoline and 10% alcohol, gasohol
burns more cleanly that gasoline alone and provides a promising method for using
renewable resources (plant material) to extend the availability of a
nonrenewable resource (petroleum). Furthermore, the alcohol needed for this
product can be obtained from the fermentation of agricultural and municipal
wastes.
The applications of fermentation span a wide spectrum, from medicines that go
into people's bodies to the cleaning of waters containing human waste. Some
antibiotics and other drugs are prepared by fermentation: for example,
cortisone, used in treating arthritis, can be made by fermenting a plant steroid
known as diosgenin. In the treatment of wastewater, anaerobic, or
non-oxygen-dependent, bacteria are used to ferment organic material. Thus, solid
wastes are converted to carbon dioxide, water, and mineral salts.
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