(a)
Interpretation:
A curved-arrow mechanism for the interconversion of 3-phosphoglycerate to 2-phosphoglycerate is to be drawn.
Concept introduction:
The interconversion of 3-phosphoglycerate to 2-phosphoglycerate is carried by enzyme catalysed nucleophilic substitution. The nitrogen of histidine residue in a protein acts as a nucleophile whereas the phosphorus of phosphate ester acts as an electrophile.
Answer to Problem 25.35AP
Explanation of Solution
The interconversion equation of 3-phosphoglycerate to 2-phosphoglycerate and the structures of histidine and phosphohistidine residue is shown below:
A curved-arrow mechanism for the interconversion of 3-phosphoglycerate to 2-phosphoglycerate is shown below:
The nitrogen of histidine residue acts as a nucleophile after loss of proton by a base A- and forms substitution product with inversion of configuration at phosphorous. In second step the nucleophilic attack of oxygen from acetate group gives 2-phosphoglycerate where the configuration at the phosphorous reverts back to original.
The two step curved arrow mechanism for the interconversion of 3-phosphoglycerate to 2-phosphoglycerate is drawn.
(b)
Interpretation:
If the isotopically chiral 3-phosphoglycerate formed would have the same configuration, the opposite configuration, or equal amounts of two configurations at phosphorus when an enantiomerically pure 2-phosphoglycerate derivative containing isoptopically chiral phosphate group is converted to 3-phosphoglycerate is to be determined.
Concept introduction:
The enzyme catalyzed substitution reaction of phosphate ester derivate at phosphate proceeds through SN2 reaction with inversion of stereochemistry. If substitution occurs twice that is with double inversion results in retention of stereochemistry.
Answer to Problem 25.35AP
If enantiomerically pure 2- phosphoglycerate is subjected to this reaction, the isotopically chiral 3- phosphoglycerate formed would have the same configuration at phosphorous.
Explanation of Solution
The enzyme catalyzed substitution reaction of phosphate ester derivate at phosphate proceeds through inversion of stereochemistry. The substitution occurs twice with double inversion of the product formed with retention of configuration. Thus, if enantiomerically pure 2-phosphoglycerate is subjected to this reaction, the isotopically chiral 3-phosphoglycerate formed will have the same configuration at phosphorous.
The interconversion of 2-phosphoglycerate to 3-phosphoglycerate by enzyme catalysed substitution occurs with retention of stereochemistry at phosphorus.
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