Question

15. Compare the catalytic activity of of the following enzymes: carbonic anhydrase, aspartate transcarbomylase (ATcase), restriction enzymes and myosins. (40 points) What is unique about each active site with regards to types of amino acid residues a) located in active site and interaction with substrate? b) Briefly what is the mechanism of catalysis in each active site? Are cofactors or coenzymes required by the enzymes and if so what is the role of the cofactor/coenzyme in catalysis? c) How is enzyme specificity maintained in the four active sites?

Answer 1

1. Carbonic anhydrase -

It serves as a catalyst for interconversion between carbon dioxide, water and bicarbonates . the active site contains zinc ion thus known as metalloenzymes. The active sites work in two forms in low pH its dehydration of bicarbonates and in high pH its hydration of carbon dioxide. Depending upon the location the role of enzyme changes. They maintain the acid base balance in the blood and help in transport of carbon dioxide.

For example in kidney water content of the body is regulated by bicarbonates. There are at least 14 different isoforms of carbonic anhydrase, in plants it helps to increase the concentration of carbon dioxide in chloroplast to increase carboxylation rate.

the enzyme are bound by to cysteine residues, one histidine residue and hydroxide ion. its interesting that zinc can be replaced with cadmium and its a cadmium containing carbon anhydrase both have the same mechanism of action but due the fact that cadmium is soft acid it will be more tightly bound to soft base ligands. the active site of CDCA is containing 9 amino acid and glycine residues.

2. Asprtate transcarbomylase-

this is a perfect example of allosteric regulation. This enzyme typically does not follow the enzyme kinetics by Michaelis-Menten they lie between relaxed and tensed states.the binding of substrate to catalytic unit shifts to R state, and binding of CTP to subunits shifts to T state. binding of ATP to subunits results in shift to R state.

In the allosteric domain of R, ATCase binds to nucleotides ATP,CTP/UTP. ATPpinds to high affinity sites and subsequently activates the enzymes,while with UTP and CTP combination binding leads to inhibition of ATCase activity.

3. Restriction enzymes-

These enzyme break DNA at specific sites called as restriction sites. They are classified into 5 types according t structure and cleavage site. The enzyme can be found in bacterias and archaea, and in prokaryotes they selectively. cut foreign DNAs. They recognize the specific sequence of nucleotides and start a double stranded cut in DNA its usually between 4 and 8 bases, the source of the enzyme can be E.coli, Haemophilus, staphylococcus.

4.Myosin-

myosin is a muscle protein.they are ATP dependent and responsible for contraction of muscles.