Can someone please explain to me how to calculate kcat from a Km curve ?

The information I have available to me is the Km and Vmax, the molecualr weight of my enzyme, and the slope from the std curve of my product.

kcat is just the Vmax divided by the enzyme concentration you used to do the experiment. The units end up being 1/sec.

Hope this helps!

[snip]
The information I have available to me is the Km and Vmax, the molecualr weight of my enzyme, and the slope from the std curve of my product.[/quote]

What are the units of your Vmax values?

If your Vmax is simply a raw rate (e.g. uMolar/min), then you need first to convert it to Specific Activity (SA) by dividing by the amount of enzyme in your assay to give uMolar/min/mg. Remember not to get confused here between Molar and moles. In this example the amount of enzyme is in ug so it's just a simple division. Then divide your SA by the molecular weight of your enzyme to give kcat (1/min). Divide again by 60 to give kcat (1/sec).