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Enzymes
Enzymes are biologic polymers that catalyze the chemical reactions by
increasing the rate of reactions without being changed in the overall
process.
Biomedical importance:
• Enzymes play an important role in metabolism, digestion and
therapeutics.
• The presence and maintenance of enzymes is essential for the
breakdown of nutrients to supply energy and chemical building blocks
• With the exception of catalytic RNA molecules, or ribozymes, enzymes
are proteins.
• The ability to assay the activity of specific enzymes in blood, other tissue
fluids, or cell extracts aids in the diagnosis and prognosis of disease.
• Deficiencies in the quantity or catalytic activity of key enzymes can result
from genetic defects, nutritional deficits, or toxins.
• Medical scientists address imbalances in enzyme activity by using
pharmacologic agents to inhibit specific enzymes and are investigating
gene therapy as a means to remedy deficits in enzyme level or function.
• Proteolytic enzymes augment the capacity of detergents to remove dirt
and stains.
• Enzymes play an important role in producing or enhancing the nutrient
value of food products for both humans and animals. The protease rennin,
for example, is utilized in the production of cheeses while lactase is
employed to remove lactose from milk for the benefit of persons who
suffer from lactose intolerance as a consequence of a deficiency in this
hydrolytic enzyme
Nomenclature of enzymes :
• A. Recommended name
• Most commonly used enzyme names have the suffix "-ase" attached to the
substrate of the reaction (for example, glucosidase urease, sucrase), or to a
description of the action performed (for example, lactate dehydrogenase and
adenylyl cyclase). [Note: Some enzymes retain their original trivial names,
which give no hint of the associated enzymic reaction, for example, trypsin and
pepsin.]
B. Systematic name (Classification of enzymes)
• The International Union of Biochemists (IUB) developed a system of enzyme
nomenclature in which each enzyme has a unique name and code number
that identify the type of reaction catalyzed and the substrates involved.
• Enzymes are divided into six major classes , each with numerous subgroups.
Classification of enzymes
• 1. Oxidoreductases (catalyze oxidations and reductions)
• 2. Transferases (catalyze transfer of moieties such as glycosyl, methyl, or
phosphoryl groups)
• 3. Hydrolases (catalyze hydrolytic cleavage of C—C, C—O, C—N, and other
bonds)
• 4. Lyases (catalyze cleavage of C—C, C—O, C—N, and other bonds by atom
elimination, leaving double bonds)
• 5. Isomerases (catalyze geometric or structural changes within a molecule)
• 6. Ligases (catalyze the joining together of two molecules coupled to the
hydrolysis of ATP)
•
Properties of enzymes :
• Enzymes are protein catalysts that increase the velocity of a chemical reaction,
and are not consumed during the reaction they catalyze. [Note: Some types of
RNA can act like enzymes, usually catalyzing the cleavage and synthesis of
phosphodiester bonds. RNAs with catalytic activity are called ribozymes , and
are much less commonly encountered than protein catalysts.]
• A. Active sites
• Enzyme molecules contain a special pocket or cleft called
the active site. The active site contains amino acid side chains
that create a three-dimensional surface complementary to
the substrate (Figure 5.2). The active site binds the substrate,
forming an enzyme-substrate (ES) complex. ES is converted
to enzyme-product (EP), which subsequently dissociates to
enzyme and product.
Properties of enzymes :
B. Catalytic efficiency
Most enzyme-catalyzed reactions are highly efficient, proceeding from 103 to 108
times faster than un catalyzed reactions. Typically, each enzyme molecule is
capable of transforming 100 to 1000 substrate molecules into product each
second.
Turnover number :The number of molecules of substrate converted to product
per enzyme molecule per second.
C. Specificity
Enzymes are highly specific, interacting with one or a few substrates and
catalyzing only one type of chemical reaction.
Properties of enzymes :
Holoenzyme refers to the enzyme with its cofactor (active enzyme).
Apoenzyme refers to the enzyme without its non protein portion of the
holoenzyme and its inactive.
D. Prosthetic groups
Prosthetic groups are distinguished by their tight, stable incorporation into a
protein's structure by covalent or non covalent forces for ( example, the biotin
bound to carboxylases and
the metal ions of Co, Cu, Mg, Mn, and Zn). All enzymes that contain tightly
bound metal ions are termed metalloenzymes .
Properties of enzymes :
Cofactors serve functions similar to those of prosthetic groups but bind in a transient,
dissociable manner either to the enzyme or to a substrate such as ATP. Unlike the
stably associated prosthetic groups, cofactors therefore must be present in the
medium surrounding the enzyme for catalysis to occur. The most common cofactors
also are metal ions. Enzymes that require a metal ion cofactor are termed metalactivated enzymes to distinguish them from the metalloenzymes for which metal
ions serve as prosthetic groups.
Coenzymes serve as a group transfer agents that transport many substrates from
their point of generation to their point of utilization. Association with the coenzyme
also stabilizes substrates such as hydrogen atoms or hydride ions that are unstable in
the aqueous environment of the cell. Other chemical moieties transported by
coenzymes include methyl groups (folates), acyl groups (coenzyme A), and
oligosaccharides (dolichol).
Properties of enzymes :
E. Regulation
Enzyme activity can be regulated, that is, enzymes can be activated or
inhibited, so that the rate of product formation responds to the needs of the
cell.
F. Location within the cell
Many enzymes are localized in specific organelles within the cell. Such
compartmentalization serves to isolate the reaction substrate or product from
other competing reactions. This provides a favorable environment for the
reaction, and organizes the thousands of enzymes present in the cell into
purposeful pathways.