This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Techniques in molecular biology, Subject name: Biochemistry
Which of the following technique is used to find out the impoant amino acid residue involved in the enzyme catalysis? 
Select and explain. 
A. Electrophoresis, B. Chromatography, C. Mass spectrometry, D. Site directed mutagenesis
D: Site directed mutagenesis
Answer: D: Site directed mutagenesis 
Explanation: Codon for specific aminoacids can be altered using site-directed mutagenesis. Site-directed mutagenesis Identification of the specific roles of given aminoacyl residues in substrate binding and catalysis. For eg : A paicular aminoacyl residue functions as a general acid can be tested by replacing it with an aminoacyl residue incapable of donating a proton.