ATPase and temperature

Hello everyone!
I am currently studying temperature and substrate effects on enzymes.

I was wondering how substrate concentration affects an
enzyme at low temperatures. A very low activity of an enzyme at low temperatures can only go up to it's Vmax.

So what about experiments to determine activity with specific substrates?
Example:
http://www.ncbi.nlm.nih.gov/pubmed/215034
What if more ATP was added at the above experiment?

Also, I was reading a book about organs and cells at low temperatures, and it writes:

"even if an adequate reserve of ATP is maintained, the membrane pumps are unable to utilize ATP at low temperatures. When the temperature returns to normal the pumps are again able to use ATP
and quickly restore the cell’s ionic gradients"

Why is that? More ATP concentration does not affect activity?


Thanx