ENZYMES

• Reaction catalysts of biological systems are called enzymes. They are the most remarkable and highly specialized proteins. They have extraordinary catalytic power, often far greater than that of synthetic or inorganic catalysts.
• Almost every biochemical reaction is catalyzed by an enzyme. They have a high degree of specificity for their substrates, they accelerate chemical reactions tremendously and they function in aqueous solution under very mild conditions of temperature and pH=7 (37 C).
• They catalyze the hundreds of stepwise reactions that degrade nutrient molecules, conserve and transform chemical energy and make biological macromolecules from simple precursors.

INTRODUCTION:

• Biological catalysts was first recognized and described in the late 1700s, in studies on the digestion of meat by secretions of the stomach.
• In the 1850s, Louis Pasteur concluded that fermentation of sugar into alcohol by yeast is catalyzed by "ferments". He postulated that these ferments were inseparable from the structure of living yeast cells; this view called vitalism.
• Then in 1897, Edward Buchner discovered that yeast extracts could ferment sugar to alcohol, proving that fermentation was promoted by molecules that continued to function when removed from the cells.
• Frederick W. Kuhne  gave the name enzymes to the molecules detected by Buchner.
• The isolation and crystallization of Urease by James Sumner in 1926 was a breakthrough. He found that urease crystals consisted entirely of proteins, and he postulated that all enzymes are proteins.
• John Northrop and Moses Kunitz crystallized pepsin, trypsin and other digestive enzymes and found them also to be proteins.
• J.B.S Haldane wrote a treatise entitled "Enzymes".

MOST ENZYMES ARE PROTEINS:

With the exception of a small group of catalytic RNA molecules, all enzymes are proteins. Their catalytic activity depends on the integrity of their native protein conformation. If any enzyme is denatured or dissociated into its subunits, catalytic activity is usually lost. If any enzyme is broken down into its component amino acids, its catalytic activity is always destroyed. Thus primary, secondary, tertiary and quaternary structures of protein enzymes are essential to their catalytic activity.

PROPERTIES OF AN ENZYME:

• Enzymes, like other proteins, have mol. wt. ranging from about 12,000 to more than 1 million.
• Some enzymes require no chemical groups for activity other than their amino acids residues.
• Other requires an additional chemical components called a cofactor, either 1 or more inorganic ions, such as Fe2+, Mg2+, Mn2+ or Zn2+.
• Or they require a complex organic or metallo-organic molecule called a coenzyme. Coenzymes act as transient carriers of specific functional groups.
• Some inorganic ions that serve as cofactors for enzymes: E.g., Cu2+ in cytochrome oxidase, Fe2+ or Fe3+ in peroxidase, catalase. Ni2+ in Urease. Se in Glutathione peroxidase. K+ in Pyruvate kinase, Mg2+ in hexokinase, glucose-6-Phosphatase. Mo in Dinitogenase.
• Some coenzymes that serve as transient carriers of specific atoms or functional groups: 1) Biocytin transfer CO2 derived from biotin. 2) Thiamin pyrophosphate transfer aldehyde 3) Pyridoxal phosphate transfers amino acids.
• Some enzymes requires both coenzymes and one or more metal ions for activity.
• A coenzyme or metal ion that is very tightly or even covalently bound to the enzyme protein is called a prosthetic group.
• A complete catalytically active enzyme together with its bound coenzyme and/or metal ions is called a holoenzyme.
• The protein part of such an enzyme is called the apoenzyme or apoprotein.
• Some enzyme proteins are modified covalently by phosphorylation, glycosylation and other processes.