Enzymes Definition : General Properties, Nature of Enzymes and All Explain About Enzymes
General Properties
Enzymes are protein catalysts for chemical reaction in biological systems. They increase the rate of chemical reactions taking place within living cells with out changing themselves.
Nature of Enzymes
Most enzymes are protein in nature. Depending on the presence and absence of a nonprotein component with the enzyme enzymes can exist as, simple enzyme or holoenzyme
1. Simple enzyme: It is made up of only protein molecules not bound to any nonproteins.
Example: Pancreatic Ribonuclease.
2. Holo enzyme is made up o protein groups and non-protein component.
-The protein component of this holo enzymes is called apoenzyme
-The non-protein component of the holo enzyme is called a cofactor.
If this cofactor is an organic compound it is called acoenzyme and if it is an inorganic groups it is called activator. (Fe 2+, Mn 2+, or Zn 2+ ions).
If the cofactor is bound so tightly to the apoenzyme and is difficult to remove without damaging the enzyme it is sometimes called a prosthetic group
COENZYMES
Coenzymes are derivatives of vitamins without which the enzyme cannot exhibit any reaction. One molecule of coenzyme is able to convert a large number of substrate molecules with the help of enzyme.
-Coenzyme accepts a particular group removed from the substrate or donates a particular group to the substrate
-Coenzymes are called co substrate because the changes that take place in substrates are complimentary to the changes in coenzymes.
-The coenzyme may participate in forming an intermediate enzyme-substrate complex
Example: NAD, FAD, Coenzyme A
Metal ions in enzymes
Many enzymes require metal ions like ca2+, K+, Mg2+, Fe2+, Cu2+, Zn2+, Mn2+ and Co2+
for their activity.
Metal-activated enzymes-form only loose and easily dissociable complexes with the metal and can easily release the metal without denaturation. Metalloenzymes hold the metal tightly on the molecule and do not release it even during extensive purification.
Metal ions promote enzyme action by
a. Maintaining or producing the active structural conformation of the enzyme (e.g. glutamine synthase)
b. Promoting the formation of the enzyme-substrate complex (Example: Enolase and carboxypeptidase A.)
c. Acting as electron donors or acceptors (Example: Fe-S proteins and cytochromes)
d. Causing distortions in the substrate or the enzyme Example: phosphotransferases).
Read Also : Introduction To Biochemistry : Amino acids and Proteins, Integrative Metabolism and Bioenergetics, Lipids and Hormones
Properties of Enzyme
A. Active site
Enzyme molecules contain a special pocket or cleft called the active site. The active site contains amino acid chains that create a three-dimensional surface complementary to the substrate.
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.
For the combination with substrate, each enzyme is said to possess one or more active sites where the substrate can be taken up.
The active site of the enzyme may contain free hydroxyl group of serine, phenolic (hydroxyl) group of tyrosine, SH-thiol (Sulfhydryl) group of cysteine or imindazolle group of histidine to interact with there is substrates.
It is also possible that the active site (Catalytic site) is different from the binding site in which case they are situated closely together in the enzyme molecule.
B. Catalytic efficiency / Enzyme turnover number
Most enzyme- catalyzed reactions are highly efficient proceeding from 103 to 108 times faster than uncatalyzed reactions. Typically each enzyme molecule is capable of transforming 100 to 1000 substrate molecule in to product each second.
Enzyme turn over number refers to the amount of substrate converted per unit time (carbonic anhydrase is the fastest enzyme).
C . Specificity
Enzymes are specific for their substrate. Specificity of enzymes are divided into:
a. Absolute specificity:- this means one enzyme catalyzes or acts on only one substrate. For example: Urease catalyzes hydrolysis of urea but not thiourea.
b. Stereo specificity- some enzymes are specific to only one isomer even if the compound is one type of molecule:
For example: glucose oxidase catalyzes the oxidation of β-D-glucose but not α-Dglucose, and arginase catalyzes the hydrolysis of L-arginine but not D-arginine.
*Maltase catalyzes the hydrolysis of α- but not β –glycosides.
Bond Specificity
* Enzymes that are specific for a bond or linkage such as ester, peptide or glycosidic belong to this group
Examples:
1. Esterases- acts on ester bonds
2. Peptidases-acts on peptide bonds
3. Glycosidases- acts on glycosidic bonds.
D. Regulation
Enzyme activity can be regulated- that is, enzyme can be, activated or inhibited so that the rate of product formation responds to the needs of the cell.
E . Zymogens (- inactive form of enzyme)
Some enzymes are produced in nature in an inactive form which can be activated when they are required. Such type of enzymes are called Zymogens (Proenzymes).
Many of the digestive enzymes and enzymes concerned with blood coagulation are in this group
Examples: Pepsinogen - This zymogen is from gastric juice.When required Pepsinogen converts to Pepsin
Trypsinogen - This zymogen is found in the pancreatic juice, and when it is required gets converted to trypsin.
* The activation is brought about by specific ions or by other enzymes that are proteolytic.
Pepsinogen + H+ Pepsin
Trypsinogen Enteropeptidase Trypsin
Zymogen forms of enzymes a protective mechanism to prevent auto digestion of tissue producing the digestive enzymes and to prevent intravascular coagulation of blood.
F. Isoenzymes (Isozymes)
These are enzymes having similar catalytic activity, act on the same substrate and produces the same product but originated at different site and exhibiting different physical and chemical characteristics such as electrophoretic mobilities, amino acid composition and immunological behavior.
Example: LDH (Lactate dehydrogenase) exists in five different forms each having four polypeptide chains. H= Heart and M=Muscle.
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