What is Km?

Memory Hack💡

High Km, LOW Affinity

Low Km, HIGH affinity

Remember that whatever affinity/Km it has, the opposite will be true for the other

Notes

Km is the value of the concentration of substrate at ½ of the Vmax. So, the amount of substrate needed to saturate ½ of the enzymes.

Km tells us a bit about the ES (enzyme-substrate) affinity: 

At a HIGH Km, there is a low affinity. 

• This Km value tells us that it will take a LOT of substrate present for the enzymes to be associated with them at ½ their maximum value. This means that they do not have a strong affinity for each other, as they are not strongly associating. 

• We can think of this as a “stickiness factor”. Say that enzymes and substrates stick together with velcro (affinity). If all of the substrates only have a little bit of velcro, they are not going to stick well to the enzymes, and therefore it's going to take a lot more chances for one of the substrates to stick→ so you need a higher amount of substrate, aka the higher Km. 

At a LOW Km there is a high affinity. 

• At low Km, this means that it will take a little bit of substrate for the enzymes to associate with them at ½ their max value. This means that they must have a high affinity, as they are strongly associated with each other. 

• To stick with our velcro example, here, the substrate has a bunch of velcro, and just one or two tries will get it to stick really well to the enzyme. Therefore, you do not need as many chances, and therefore less substrate concentrations. 

Additionally, Km is useful in nature. This is where the body typically likes to keep its levels of substrate and enzymes. 

...Why? 

You just need to flux the concentration of substrate up a little bit to speed things up, and down just a bit to slow them down. Nothing drastic, just the way the body likes it! 

Concept Check: 

Why is a low Km good for a scarce substrate? 

Explanation: If you have a substrate that is not very available, you want it to bind with a HIGH affinity for the enzyme.