Enzyme kinetics fundamentals & terms: Km, kcat, specificity constant/catalytic efficiency, activity
Much on Michaelis-Menten kinetics: https://bit.ly/maudmenten ; YouTube: • Michaelis-Menten kinetics - giving en...
But, key points:
Km – the Michaelis-Menten constant. The concentration of substrate at which you get 1/2 maximum velocity. Does NOT depend on enzyme or substrate concentration. It's a constant (for the conditions). It roughly corresponds to affinity. The weaker the affinity, the higher the Km and vice versa
kcat – the turnover number. How quickly an enzyme converts substrate to product if it's saturated with substrate. It's the catalytic rate constant (or a simplification of it). It also doesn't depend on substrate concentration. Because it, too, is an intrinsic thing.
vmax – the maximum velocity. How quickly a bunch of enzymes convert substrate to product if they're saturated with substrate. You figure it out by measuring the initial velocity (vo) of an enzyme with different amounts of substrate (so graphing time vs. product formation), then plotting those vo's vs. substrate concentration and seeing where it maxes out.
Divide the vmax by the enzyme concentration to get the kcat.
kcat/Km is the specificity constant. It tells you how "good" an enzyme is towards a substrate by taking into account how well it binds and how fast it converts the substrate to product. The bigger, the better. Sometimes it's called the catalytic efficiency, but that can be misleading because it can only really tell you about which substrates an enzyme is better at handling, not which enzymes are better at handling a substrate.
Activity is how much gets done how fast. It's often given in terms of activity units (U), which are defined for that enzyme (e.g. mol converted per minute or something). It does NOT take into account how much enzyme was doing all that doing
Specific activity DOES take into account enzyme concentration. But it really takes into account TOTAL protein concentration. You divide the activity by the mg protein to get specific activity. The more active the enzyme the higher that will be, but the less active OR less pure the enzyme, the lower it will be. It's good for comparing between different samples of the same enzyme or different versions of the same enzyme. Plus, you can get this value much easier than finding all the kinetic constants!
More on specific activity: http://bit.ly/enzymeactivity ; YouTube: • Specific activity & enzyme activity u...
interesting article on Km: The meaning of the Michaelis-Menten constant: Km describes a steady-state. Enric I. Canela, Gemma Navarro, José Luis Beltrán, Rafael Franco. bioRxiv 608232; doi: https://doi.org/10.1101/608232
more about all sorts of things: #365DaysOfScience All (with topics listed) 👉 http://bit.ly/2OllAB0 or search blog: http://thebumblingbiochemist.com
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