You will have to be more specific with your question.

Do you want to monitor NAD binding to GAPDH in vitro?

Do you want to model NAD binding in silico?

Are you asking what isoform of GAPDH to use?

 I want monitor NAD binding to GAPDH in vitro

Knowing why you want to do this might help any suggestions.

You could:

Monitor binding of radio-labeled NAD using a radio-ligand binding assay.

Monitor binding of unlabelled NAD using a thermal shift assay.

Monitor binding of NAD indirectly by performing classical bi-substrate enzyme kinetics experiments.

Which one do you think would be best?

I was doing enzyme kinetics experiments... 

You can give a piece of advice if you can ...