Hey all, today’s blog post will be on propositions of both the Michaelis-Menten curve and the Lineweaver Burk plot. I find this to be the most difficult sub-topic in the entire enzymes topic, so I thought it would be useful to post on it, to help any other students having difficulty.
The Michaelis-Menten curve was proposed by Lenor Michaelis & Maude Menten and accounts for most of the features of enzyme-catalyzed reactions. This model shows the enzyme reversibly combining to its substrate to form an enzyme substrate complex that forms products and regenerates the free enzyme. So for the course we are not required to derive the Michaelis-Menten equation but we are required to know the assumptions. These assumptions are just to be learnt off, so here they are:
1. Relative concentration of enzyme & substrate
The substrate concentration is much greater than the enzyme concentration, so that the percentage of total substrate bound by the enzyme at any one time is small.
2. Steady state assumption
Enzyme-substrate concentration does not change with time that is the rate of formation of enzyme substrate is equal to that of the breakdown of enzyme substrate. In general, an intermediate in a series of reactions is said to be in steady state when its rate of synthesis is equal to its rate of degradation.
3. Initial velocity
Initial reaction velocities are used in the analysis of enzyme reactions. This means that the rate of the reaction is measured as soon as enzyme and substrate are mixed. At that time, the concentration of product is very small and therefore, the rate of the back reaction from P to S can be ignored.
Conclusions of the Michaelis-Menten Curve
· Km is numerically equal to the substrate concentration at which the reaction velocity is equal to ½ Vmax.
· Km does not vary with concentration of enzyme
· Small Km = High affinity of enzyme for substrate
· Large Km = Low affinity of the enzyme for substrate
Refer to graph below.
The effect of substrate concentration on reaction velocity for an enzyme catalyzed reaction
From looking at the graph you can see that at low concentrations of substrate < Km, the velocity of the reaction is first order that is it is proportional to substrate concentration. We can also observe that at high concentrations the substrate concentration > Km, the velocity of the reaction is zero order that is it is constant and independent of substrate concentration. The rate of reaction is directly proportional when the substrate concentration is much less than the Km value.
The Lineweaver-Burk plot also known as the double reciprocal plot can be used to calculate Km and Vmax as well as determine the mechanism of action of enzyme inhibitors. In this plot we see that 1/vo is plotted versus 1/[S] and a straight line is obtained.
See graph below.
The graph shows that the intercept on the x-axis is equal to -1/Km and the intercept on the y-axis is equal to 1/Vmax.
I hope this post assisted some of you. Thanks for following my blog. Enjoy the rest of your day.