Enzymes MCQs for Nurses
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Basic concept about enzyme and co-enzymes includes activation energy, holoenzyme, apo-enzyme, co-factors and metal ions.
Enzymes ans Substrates
Enzymes are another important group of biomolecules synthesized by the living cells. They are catalysts of biological systems (hence are called as biocatalysts), colloidal, thermolabile and protein in nature. They are remarkable molecular devices that determine the pattern of chemical transformations.
They also mediate the transformation
of different forms of energy. The striking characteristics of enzymes are their
catalytic power and specificity. Actions of most enzymes are under strict
regulation in a variety of ways. Substances on which enzymes act to convert
them into products are called substrates.
What is Activation Energy
Catalytic Activity of Enzymes: Enzymes have immense catalytic power and accelerate reactions at least a million times, by reducing the energy of activation . Before a chemical reaction can occur, the reacting molecules are required to gain a minimum amount of energy, this is called the energy of activation . It can be decreased by increasing the temperature of the reaction medium. But in human body which maintains a normal body temperature fairly constant, it is achieved by enzymes.
Holoenzyme , Apoenzyme and Co-Enzymes
Protein Nature of Enzymes: In general with the exception of ribozymes which are few RNA molecules with enzymatic activity, all the enzymes are protein in nature with large mol. wt. Few enzymes are simple proteins while some are conjugated proteins. In such enzymes the non-protein part is called prosthetic group or coenzyme and the protein part is called as apoenzyme . The complete structure of apoenzyme and prosthetic group is called as holoenzyme.
Holoenzymes = Apoenzymes (Protein part) + Coenzymes (Prosthetic group)
Protein chains in enzymes
Certain enzymes with only one polypeptide chain in their structure are called as monomeric enzymes , e. g. ribonuclease . Several enzymes possess more than one polypeptide chain and are called as oligomeric enzymes , eg lactate dehydrogenase, hexokinase, etc. Each single polypeptide chain of oligomeric enzymes is called as subunit .
When many different enzyme catalysing
reaction sites are located at different sites of the same macromolecule, it is
called as multienzyme complex . The complex becomes inactive when it is
fractionated into smaller units each bearing individual enzyme activity, eg
fatty acid synthetase, carbamoyl phosphate synthetase II,pyruvate
dehydrogenase, prostaglandin synthase, etc.
Coenzymes
Certain enzymes require a specific, thermostable, low mole. w t , non-protein organic substance called as coenzyme . A coenzyme may bind covalently or noncovalently to the apoenzyme. The term prosthetic group denotes a covalently bonded enzyme. It is generally observed that reactions involving oxidoreductions, group transfers, isomerization and covalent bond formation require coenzyme .
Since the
involvement of coenzyme in a given reaction on a substrate is so intimate that
coenzyme is often called as cosubstrate or second substrate. Many
coenzymes are derived as the physiologically active forms from the constituents
of vitamin B-complex viz, Pantothenic acid: CoASH , Vitamin B12: Cobamide ,
Folic Acid: Tetrahydrofolate (F.H4), Niacin: NAD+, NADP, Riboflavin: FMN, FAD ,
Pyridoxine: Pyridoxal phosphate, Thiamine: TPP.
Classification of Coenzymes
Coenzymes can be classified
according to the group whose transfer they facilitate . Based on this
concept we may classify coenzymes as follows:
1:For transfer of groups other than hydrogen
2:For transfer of hydrogen
NAD+, NADP+,FMN, FAD,Lipoic acid and Coenzyme Q.
In addition heme acts as coenzyme in
cytochromes, peroxidases and PG synthase complex. Many coenzymes contain
adenine, ribose and phosphate and are derivatives of adenosine monophosphate
(AMP) such as NAD, FAD.
Role of metal ions in enzymes
The activity of many enzymes depends
on the presence of certain metal ions such as K+, Mg++, Ca++, Zn++, Cu++.
1:Metal activated enzymes: In
certain enzymes the metals form a loose and easily dissociable complex. Such
enzymes are called metal-activated enzymes .The metal ions can be
removed by dialysis or any other such method from the enzyme without causing
any
denaturation of apoenzyme.
2:Metalloenzymes: The second
category of metal enzymes is called as metalloenzymes . In this case metal
ion is bound tightly to the enzyme and is not dissociated even after
several extensive steps of purification.
Reference:
Notes Made By The Help of "The Text Book of Medical Biochemistry By MN. Chatterjea 8th Edition"
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