ENZYMES

Some major properties of enzymes are:

• They are proteins which act as catalysts
• A catalyst acts to speed up a chemical reaction without being altered itself during the chemical reaction. This means that enzymes can be recycled and do not need to be made in large amounts.
• Enzymes lower the activation energy needed to drive a biochemical reaction and without a need for increase in temperature. In this manner, cellular proteins are not damaged by excess heat from reactions.
• They are generally globular proteins with distinct 3D forms known as a "native conformation."
• The native conformation of an enzyme is essential for its functional activity. The native conformation of an enzyme is maintained by hydrogen and disulfide bonds. Disruption (breakage) of these bonds leads to a loss of enzyme function known as denaturation and typically involves loss of the active site of an enzyme.
• Denaturation can occur at high temperatures (typically over 50 C) and because of large shifts in pH. Heavy metal compounds such as silver nitrate and mercurochrome (containing mercury) act to control bacterial growth by denaturing their proteins. Alcohols serve the same function.
• Enzymes function at an optimal and narrow pH range. The majority of enzymes found in biological systems function best at and around neutral pH (between pH 6.8 an pH 7.4)
• There are exceptions to every rule however. There are plenty of microbes with enzymes that can resist extreme pH conditions and high temperatures. An interesting example is a DNA polymerase isolated from the bacterium Thermus aquaticus. This organism lives in volcanic hotsprings and its DNA polymerase can withstand temperatures well above 50 C. This enzyme has been put to use in a now commonplace procedure for amplifying DNA; the so called polymerase chain reaction (PCR).
• All enzymes have a characteristic turnover number which is the number of substrate molecules converted to product per second. One of the fastest known enzymes is catalase which converts up to 500,000 hydrogen peroxide molecules per second into water and oxygen. You may be familiar with the action of catalase when you treat a cut or a "zit" with hydrogen peroxide and you notice the production of bubbles due to release of oxygen.

ENZYMES ARE NAMED ACCORDING TO THE CHEMICAL REACTION THAT THEY PARTICIPATE IN

• The virulence of many bacteria is thought to be contributed to by secreted "exoenzymes." Such enzymes can help bacteria digest cells and tissues and enable microbesto spread around the body

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THE LOCK AND KEY HYPOTHESIS OF ENZYME ACTION

• Enymes are highly specific for a given substrate which is due to the shape of the active site. Each substrate (key) has a specific conformation that can fit into the active site of the enzyme (lock).
• Holoenzymes consist of a protein portion (apoenzyme) and nonprotein portion (cofactor). Enzymes are inactive in the absence of a cofactor.

Cofactors generally fall into two major categories:

• Metal or mineral ions:
• Most trace elements used by living cells are primarily used to activate cellular enzymes. Such elements include Mg, Mn, Zn, and Cu. For example, Mg is needed for DNA polymerase to form a growing DNA molecule.
• metal cofactors help form a bridge between enzyme and substrate and stabilize reaction or help maintain shape of active site.
• complex nonprotein organic molecules called coenzymes
• Many important cofactors are derived from the vitamin B complex (8 substances)
• Three coenzymes are particulary important in cellular respiration and act as temporary electron (energy carriers)

ENZYME INHIBITION

There are three major forms of enzyme inhibition

Feedback/end-product inhibition . The end product of a metabolic pathway typically inhibits an enzyme acting at the beginning of a pathway.

Competitive inhibition

Competitive inhibitors compete with a substrate for the active site of an enzyme due to similarities in structure. Such inhibition can generally be reversed if the substrate concentration is higher than that of the inhibitor.

A well known example of competitive inhibition in microbiology is the suppression of microbial growth by sulfa drugs. The compound known as PABA is an essential nutrient for many bacteria to make folic acid. Folic acid is a coenzyme needed for the synthesis of purine and pyrimidine bases in nucleic acids. Sulfa drugs compete with PABA and block its conversion into folic acid and consequently interfere with the DNA replication of bacteria. Humans do not need PABA to make folic acid and therefore are not directly affected by the use of sulfa drugs in this manner (although some people are extremely allergic to these drugs).

Noncompetetive/allosteric inhibition

Noncompetitive inhibitors are substances that bind to a location other (allosteric) than the active site of an enzyme. However, such binding generally distorts the enzyme active site rendering it nonfunctional. Certain enzyme poisons act in this manner such as cyanide which binds to iron cofactors in iron-containing enzymes.