Short Notes On Basics of Isozymes and Zymogen, Download PDF!

By Renuka Miglani|Updated : July 25th, 2022

Are you an Aspirant of CSIR-NET and looking for some short and reliable notes for Life Sciences to strong your base for preparations? We have got you covered!

Candidates preparing for their CSIR NET exam can make their preparation journey easier with the help of reliable study notes that cover the topics most simply. We at BYJU'S Exam Prep have come up with the idea of providing short notes on Isozymes and Zymogen, which come under the Biochemistry section of the Life Science syllabus. 

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Short Notes On ISOZYME & ZYMOGEN

ISOZYMES:

Isozymes (also known as isoenzymes) are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. They are encoded by different genes located at different loci. These enzymes usually display different kinetic parameters or different regulatory properties. Isozymes differ from allozymes, which are enzymes that arise from alternative forms of the same gene, i.e., alleles. A common example of an isozyme is lactate dehydrogenase (LDH) which catalyzes the reversible conversion of pyruvate to lactate. Human beings have two isozymic polypeptide chains for this enzyme: H polypeptide and M polypeptide. The functional enzyme is tetrameric (homo- or heterotetramer). Two different subunits combine randomly with each other forming five isozymes. The kinetic properties of the various LDH isozymes differ in terms of their relative affinities for the various substrates and their sensitivity to inhibition by product. Different tissues express different isozyme forms, as appropriate to their particular metabolic needs. 

ISOZYMENATURE OF SUBUNITSLOCATION
LDH1HHHH (H4)

In heart and RBCs

LDH2HHHM (H3M)

In the reticuloendothelial system

LDH3HHMM (H2M2)

In the lungs 

LDH4

HMMM (HM3)

In the kidney, placenta

LDH5

MMMM (M4)

In the liver and striated muscle

Similarly, there are four important mammalian hexokinase isozymes that vary in subcellular locations and kinetics concerning different substrates and conditions. They are designated as hexokinase I, II, III, and IV or hexokinase A, B, C, and D. Hexokinase catalyzes the phosphorylation of hexose (such as glucose). While hexokinase I, II, and III are capable of phosphorylating several hexoses, hexokinase IV (also referred to as glucokinase) acts only on glucose as hexose. Glucokinase is mainly expressed in liver and pancreatic 𝛃-cells and is characterized by a low affinity for its substrate glucose. Hexokinase I, II, and III have a high affinity for glucose even at low concentrations, and all three are strongly inhibited by their products glucose-6-phosphate. Glucokinase can only phosphorylate glucose if the concentration of this substrate is high enough.

 

ZYMOGEN:

An inactive precursor of an enzyme is called zymogen. Zymogen is cleaved to form the active enzyme. Many proteolytic enzymes like chymotrypsin, trypsin are initially synthesized as inactive precursor chymotrypsinogen and trypsinogen. Specific cleavage causes conformational changes in the inactive precursor that expose the enzyme active site. In the duodenum, the pancreatic zymogens, trypsinogen, chymotrypsinogen, proelastase, and procarboxypeptidase are converted into active enzymes by enteropeptidase and trypsin. 

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Chymotrypsinogen, a single polypeptide chain of 245 amino acid residues, is converted to a-chymotrypsin, which has three polypeptide chains linked by two of the five disulfide bonds present in the primary structure of chymotrypsinogen. 

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