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# Amino Acids at the Active Site

BIOCHM-0\$UTYB

Arginosuccinate synthetase catalyzes the reaction shown below.

$$\text{citrulline + aspartate + ATP} \xrightarrow [ ]{ } \text{argininosuccinate + AMP + pyrophosphate}$$

In the course of the reaction, an arginine at the active site plays a role in helping to bind the negatively charged ATP and aspartate. A reduction of activity by this enzyme leads to the condition of citrullinemia.

A series of mutants were created by mutating the gene that encodes this enzyme and the following was observed: mutant A which changed arginine at position 139 to lysine had 95% of the original activity but mutant B which changed arginine at position 139 to glutamate had 5% of the activity.

Meanwhile, mutant C had an arginine at position 23 changed to lysine and had 95% of the original activity while mutant D in which the arginine at position 23 was changed to glutamate had 95% of the original activity.

Which of the following choices is a reasonable explanation of these results?

A

The arginine at position 23 is at the active site and it can be replaced by any amino acid with no loss of activity.

B

The arginine at position 139 is at the active site and activity of this enzyme requires a positively charged amino acid residue at that location.

C

The arginine at position 23 is at the active site and the activity of this enzyme requires a positively charged amino acid residue at this position.

D

The arginine at position 139 is at the active site and the active site will be functional regardless of which amino acid is present.

E

There is not enough information to tell which amino acid is at the active site.