Some investigators modified the Cys 278 with iodoacetamide and found that the en

Question

Some investigators modified the Cys 278 with iodoacetamide and found that the enzyme activity was abolished as a result. They concluded that the Cys 278 residue was absolutely essential for enzymatic activity. But other investigators modified the Cys 278 with iodoacetate and found that activity was decreased but not abolished, leading them to conclude that the Cys 278 was not essential. Taking these results together with the results of the current study, can you suggest an explanation that will clear up the confusion?

Explanation / Answer

Cys 278 modified with iodoacetamide has no activity.
Cys 278 modified with iodoacetate has reduced enzyme activity.
It indicates that the amino group of idoacetamide forms an intermediate by reacting with carbonyl group of Cys 278 and makes it enzyme inactive.
However no such intermediate is formed between Cys 278 and iodoacetate. Hence, the activity is not abolished.

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