this chymotrypsin and trysin have similar structure sequentially have similar function.there can cut protein chain. All three have a ser at 195, his at 57, Asp at 102
Single polypeptide don’t have catalytic activity.
What are enzymes?Enzymes are proteins which act as catalysts Catalyst : A catalyst is something which by its very nature increases the rate of a reaction and remain uncharged at the end of reaction.
Oligomeric enzymescontains two or more polypeptide chains associated bynoncovalent forces. Oligomeric enzyme 14
Zymogen Several enzymes are produced and stored asinactive precursors called zymogens IsozymesEnzymes which catalyze the same reaction buthave different primary and quaternary structure.The most common isozymes are polymericenzymes.May have similar but not identical amino acidsequences.May have common evolutionary origin. 15
Oligomeric enzymes consist of two or morepolypeptide chains which are usually linked toeach other by non covalent interactions andnever by peptide bonds. The componentpolypeptide chains are termed sub-units andmay be identical ,they are some times calledprotomers. Dimeric proteins consist oftwo,trimeric proteins of three and tetramericproteins of four sub-units. The molecularweight is usually in excess of 35000.
The vast majority of known enzymes areoligomeric for example ,all of the enzymesinvolved in glycolysis process either two or foursub-units. It is therefore, reasonable to assumethat the sub-units of oligomeric proteins inassociation that they do not have in isolation.such enzymes are not synthesized as inactivezymogens, but their activities may be regulatedin a far more precise way by feed-backinhibition. This is possible because manyoligomeric proteins exhibit allostery theirdifferent binding sites interact.
LACTATE DEHYDROGENSE Vertebrate LDH is an ex of oligomeric enzymewhere each subunit has the same function ,in this case to catalise the reaction CH3.CH.CO2 + ־NAD+ CH3.C=O.CO2־ | || PYRUVATE OH O (S)-LACTATE + NADH + H+
e.g. LDH(lactate dehydrogenase) LDH1,2,3,4,5 are HHHH, HHHM, HHMM,HMMM and MMMM . i.e. LDH isozymesare tetramers formed by 2 sets of subunits. 19
Significance : In the heart, LDH1 (4 H) H=heart, catalyzes Lactateconvert to pyruvate for energy supply In muscles, LDH5 ( 4M ) M=muscle, convert pyr toLact. For energy storage Clinical diagnosis using isozyme. E.g. when heartattack(infarction) happens, enzymes release frominjured cells to the blood showed differentenzyme(isozyme ) pattern. Isozyme pattern: different isozymes appear as apeak sooner or later followed by the progress of thedisease. 22
LACTSOSE SYNTHASE• Mammary gland lactose synthase is an example of oligomeric enzyme where a non functional sub unit modifies the behavior of a functional sub unit. This enzyme as isolated from milk , consist of two subunits: one of these catalytically inactive protein , α-lactalbumin , found only in mammary gland; the other is N-acetyl lactosaminesynthase, an enzyme present in most tissues .• is N-acetyl lactosaminesynthase, in the absence of α-lactalbumin, catalyses the reaction
This is important in the synthesis of the carbohydratecomponents of glycoproteins, the enzyme is alsoproduced and stored in the mammary gland duringpregnancy ,when levels of α-lactalbumin are low. After thebirth of the baby, reduced synthesis of the hormoneprogesterone in the mother leads to increased synthesisof the luteotrophic hormone (prolactin), stimulating theproduction of alpha lactalbumin in the mammary gland.This combines with the stored is N-acetyl lactosaminesynthase to form lactose synthase an enzyme whichspeculates production of the lactose components lactosesynthase, an enzyme with facilitates the production of thelactose components of the milk required for the newbornbaby. Lactose synthase catalyses the reaction:UDP-galactose + glucose UDP + lactose
TRYPTOPHAN SYNTHASE• Tryptophan synthase of E.coli is an ex of oligomeric enzyme which contains two different functional sub- units the enzyme catalyses the reaction• Indole-3-glyserolphasphate+L-serine Pyridoxal phosphate• L-tryptophan+glyceraldehyde-3-phosphate• It can be dissociated into two or further units, each of molecular weight 29000 and a beta-2 subunit, of molecular weight 90000, the beta-2 subunit further dissociates the presence of 4 M urea to give two beta subunits, each of which has a binding site for the co- enzyme pyrodoxal phosphate.• The isolated alpha-subunit will catalyze the reaction:• Indole-3-glyserolphasphate indole + glyceraldehyde-3- phosphate.• The isolated beta-subunit also has catalytic activity but for the reaction: Pyridoxal phosphate
PYRUVATE DEHYDROGENASE• The same type of organization as tryptophan synthase, but on an even larger scale. The Enzyme Commission recommended that such a complex should be regarded as a system of separate enzymes rather than as single enzyme.• Pyruvate dehydrogenase enables pyruvate to enter the TCA Cycle by, catalyzing its overall conversion to acetyl-coA:• Pyruvate + Co ASH + NAD+ acetyl– CoA +• CO2 + NADH.
several factors contribute to enzyme catalysis 1. Proximity effects and orientation arrange: 2. Electrostatic effects 3. Acid-base catalysis 4. Covalent Catalysis 27