Ka vs Km

[Astra]

Both are measures of affinity.

I would like clarifications as to how they are different.

My understanding is the following:

Km: The concentration of substrate at which half of the active sites are full.

Smaller Km indicates a higher affinity because it means you need less substrate to reach half full due to them being attracted.


Ka: [ Substrate enzyme complex] / [Enzyme concentration]

So a higher Ka would indicate that for the same enzyme concentration, the enzyme would be more attracted to the substrate.


So are Km and Ka 2 ways of approaching affinity from a substrate and enzyme concentration perspectives respectively?

Thanks for the help guys!

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[theonlytycrane]

E + S <-- k-1 / k1 --> ES -- k2 --> E + P

Km is the dissociation constant for enzyme kinetics. Km = k-1 + k2 / k1 and as you mentioned it's inversely proportional to binding affinity between the enzyme and substrate. It's also equal to substrate concentration when v = 1/2 * vmax.

Km tells you about enzyme-substrate binding affinity. A lower Km means better enzyme-substrate binding.