Enzymes

French chemist Anselme Payen was the first to discover an enzyme, diastase, in 1833. A few decades later, when studying the fermentation of sugar to alcohol by yeast, Louis Pasteur concluded that this fermentation was caused by a vital force contained within the yeast cells called "ferments", which were thought to function only within living organisms. He wrote that "alcoholic fermentation is an act correlated with the life and organization of the yeast cells, not with the death or putrefaction of the cells."

In 1877, German physiologist Wilhelm Kühne (1837–1900) first used the term enzyme, which comes from Greek ἔνζυμον, "leavened", to describe this process. The word enzyme was used later to refer to nonliving substances such as pepsin, and the word ferment was used to refer to chemical activity produced by living organisms.

Enzymes are proteins that have catalytic functions indispensable to maintenance and activity of life. All chemical reactions occurring in a living organism are dependent on the catalytic actions of enzymes, and this is why enzymes are called Biotransformation. At present, there are about 4,000 kinds of enzymes whose actions are well known.

Enzymes function in a mild environment similar to the body environment of a living organism, and they support life by synthesizing and degrading materials that constitute the building blocks of the organism and by creating energy. Enzymes function as highly selective catalysis in such a way that they selectivity catalyze specific reactions (reaction specificity) and specific materials (substrate specificity).

Most chemical catalysts catalyse a wide range of reactions. They are not usually very selective. In contrast enzymes are usually highly selective, catalysing specific reactions only. This specificity is due to the shapes of the enzyme molecules.

Many enzymes consist of a protein and a non-protein (called the cofactor). The proteins in enzymes are usually globular. The intra- and intermolecular bonds that hold proteins in their secondary and tertiary structures are disrupted by changes in temperature and pH. This affects shapes and so the catalytic activity of an enzyme is pH and temperature sensitive.

Cofactors may be:

• organic groups that are permanently bound to the enzyme (prosthetic groups)
• Cations - positively charged metal ions (activators), which temporarily bind to the active site of the enzyme, giving an intense positive charge to the enzyme's protein
• Organic molecules, usually vitamins or made from vitamins (coenzymes), which are not permanently bound to the enzyme molecule, but combine with the enzyme-substrate complex temporarily.

Classification of enzymes

Enzymes can be classified by the kind of chemical reaction catalyzed.

1.     Addition or removal of water

A.    Hydrolases - these include esterases, carbohydrases, nucleases, deaminases, amidases, and proteases

B.    Hydrases -such as fumarase, enolase, aconitase and carbonic anhydrase

2.     Transfer of electrons

A.    Oxidases

B.    Dehydrogenases

3.     Transfer of a radical

A.    Transglycosidases - of monosaccharides

B.    Transphosphorylases and phosphomutases - of a phosphate group

C.    Transaminases - of amino group

D.    Transmethylases - of a methyl group

E.    Transacetylases - of an acetyl group

4.     Splitting or forming a C-C bond

A.    Desmolases

5.     Changing geometry or structure of a molecule

A.    Isomerases

6.     Joining two molecules through hydrolysis of pyrophosphate bond in ATP or other tri-phosphate

A.    Ligases

Mechanism Theories of enzymes:

The Lock & Key theory describe a situation where the substrate fits perfectly into the enzyme’s activity site. The activity site is simply the location of the enzyme where the substrate docks.

The Induced Fit model begins with an enzyme activity site which does not exactly fit the substrate, but is stretched to accommodate the substrate. 

This second theory gives a more flexible image of an enzyme than the first.

The ability for an enzyme to dock and undock molecules is essential in increasing the rate of the reactions. This turnover number the number of substrate molecules the enzyme is able to act on in one minute. This turnover number has been standardized by the chemical community into the enzyme international number. Which is the number of catalytic conversions per minute of 1 micro mole of substrate, under specific conditions of temperature and pH.

Enzyme Activity Factors:

An enzyme catalyzed reaction may be written as:

E is the enzyme

S is the substrate (reactant)

ES is the enzyme-substrate complex

P is the product

The Equation which describes rate of reaction (r) as a function of substrate concentration (S) is the Michaelis-Menton Equation.

Vmax is the function of the concentration of the enzyme [E] and the Turnover Number of the enzyme.

Km is the Michaelis-Menton constant and is determined experimentally, just as the rate constant is for a rate law equation.

Remember, the Turnover Number refers to the efficiency of the enzyme and is expressed as the number of molecules of substrate converted to product per second.

The Turnover Number of enzymes can range from 10 to 100,000 molecules per second, demonstrating the effective catalytic nature of some enzymes.

Substrate Concentration

At low substrate concentrations, each enzyme molecule is reacting with fewer substrate molecules than is suggested by its turnover number. As the substrate concentration is increased, each enzyme is able to locate and react with more substrate molecules and the observed enzyme activity increases. 

However, once the turnover number is reached, the addition of more substrate does not further increase the rate.

Enzyme Concentration

Under normal biological conditions the substrate is present in a large excess (there is much more substrate than enzyme). As long as this condition is maintained, the addition of more enzyme results in a proportional increase in rate.

pH

Enzyme activity is influenced by pH with each enzyme having an optimum pH. The optimum pH is the pH at which the activity of a particular enzyme is at a maximum. The optimum pH's for enzymes found in the body are matched to the pH of the biological systems in which they operate. For example, pepsin, a digestive enzyme, has an optimum pH of 1.5, the pH of the stomach. 

At pH's far from the optimum pH, enzyme activity can be reduced to nearly zero. If the reduction in activity is reversible the enzyme has been denatured. If it is not reversible, the enzyme has been digested.

Temperature

Just as each enzyme has an optimum pH, each has an optimum temperature as well. Most human enzymes have an optimum temperature about that of body temperature (98.6^oF) and are denatured or digested at extreme temperatures.

Presence of Cofactors

Some enzymes are capable of catalytic activity by themselves. Others require the presence of an additional substance called a cofactor to induce this behavior. If the cofactor is an organic compound, it is called a coenzyme. If it is a metal ion, it is called a metal ion activator. If a required cofactor is not present, the catalytic activity of the enzyme is dramatically reduced.

Presence of Inhibitors

Inhibitors are substances that reduce the rate of enzyme activity, usually by binding with the enzyme and interfering with the formation of the enzyme-substrate complex. While some heavy metals act as metal ion activators, they can also act as enzyme inhibitors.

Enzyme Inhibition:

1.     Chemicals other than substrates and products may interact with an enzyme influencing the reaction rate.

2.     Chemicals which bind to the active site but do not react will compete for formation of the ES complex and are known as competitive Inhibitors. Raising substrate concentrations will overcome this type of inhibition. 

3.     Chemicals which bind somewhere else than the active site but decrease the turnover constant for the enzyme are known as non-competitive inhibitors. Raising the substrate concentration will not overcome this type of inhibition.  

4.     Some agents simply denature or otherwise destroy the enzyme causing irreversible Inhibition. This type of inhibition is see often with chemicals which form covalent bonds with the enzyme.  CN^-, cyanide ion is an example of an irreversable inhibitor.  It binds to the cytochrome oxidase, a cofactored enzyme, and prevents it from allowing cell respiration.  If Na₂S_sO₃ is administered quickly the CN^- can be removed from the cofactored enzyme.  Most heavy metals, Pb, Hg denature enzymes in a similar manner.

Not all inhibitors are bad for you.  Penicillins act as an inhibitors for the transpeptidase enzyme which builds the protein portion of bacteria cell walls. 

Cofactors:

All enzymes are proteins.  But sometimes, for a protein to function it requires the assistance of another molecule or ion, a cofactor.  Cofactors are normally thought of as metal ions, but when this cofactor is a small organic molecule it is named a coenzyme.  The magnesium ion is required for the body to use glucose-phosphate compounds.  Coenzymes are also called vitamins.  Further yet, some coenzymes themselves must by accompanied by another organic molecule to function properly.  If an enzyme requires a cofactor the protein is named apoenzyme until the cofactor has been added.

VITAMIN	FUNCTION	DEFICIENCY SYMPTOM
WATER SOLUBLE
Ascorbic Acid (C)	Hydroxylases	Scurvy - connective tissue
Thiamine (B1)	reductases	Beriberi-fatigue
Riboflavin (B2)	reductases (Flavine)	Skin disease
Pyridoxine (B6)	Aminotransferases	Anemia
Niacin	reductases (NAD)	Pellagra -skin and nerve
Folic Acid (M)	Methyltransferases	Anemia
Vitamin B12	Isomerase	Anemia
Pantothenic Acid	Acyltransferase (CoA)	Weight Loss
Biotin (H)	Carboxylases	Dermatitis
FAT SOLUBLE
Vitamin A	Vision and Cell Differentiation	Night Blindness
Vitamin D	Calcium Metabolism	Rickets - bone problems
Vitamin E	Antioxidant	Fragile cell membranes
Vitamin K	Blood Clotting	Delayed blood clotting