Biology: Chapter 3: Enzymes: Enzyme affinities

Biology: Chapter 3: Enzymes: Enzyme affinities 

Vmax: Maximum theoretical rate (velocity); all enzymes binded with a substrate. However, in theory, there needs to be an infinite substrate concentration, but this is impossible to measure.
Km (Michaelis-Menten constant): Substrate concentration at which an enzyme works at half its maximum rate (1/2 Vmax). Measure of enzyme affinity. Inverse measure; a higher Km means that more substrates need to be present for enzymes to be saturated (binded with a substrate), which means low affinity. However, a low Km means only a small amount of substrate is needed to saturate the enzymes - high affinity. So the lower the Km, the faster the reaction will proceed to its maximum rate.

• It is impossible to find the Vmax by reading results from a graph, because in reality there isn't an infinite substrate concentration. 
• However, since 1/infinity = 0, which can be plotted, Vmax can be found by plotting 1/[S] (inverse substrate concentration) on the x-axis and 1/velocity (inverse rate). This is called a double reciprocal plot.

• Vmax: On the double reciprocal graph, you can find 1/Vmax on the y-axis. To find Vmax, you can calculate it like this (let's pretend that 1/Vmax is 20 in this example): 
• 1/Vmax = 20
  1= 20 x Vmax
  Vmax = 1/20
  Vmax = 0.05
  

• Km: On the double reciprocal graph, you can find -1/Km on the x-axis. To find the Km, you can calculate it like this (let's pretend -1/Km is -10 in this example):
• -1/Km = -10
  -1= -10 x Km
  Km = -1/-10
  Km = 1/10
  Km= 0.1

• Km for enzymes can vary depending on many factors, eg temperature, pH, presence of particular ions, substrate, ion concentration and the presence of poisons, pollutants or inhibitors.

Significance of Vmax and Km values

• Helps scientists make computerized models of biochemical pathways or even the behaviour of whoe cells by predicting how each reaction in a proposed pathway will proceed, and therefore how enzymes interact, and then the consequences of changing factors such as pH and temperature can be built into models.
• By understanding what affects enzyme efficiency, scientists in the future may be able to design better catalysts, which is linked to genetic engineering.
• Knowing the Km means we can calculate active sites occupied by substrates for any substrate concentration.
• We can compare performances of different enzymes.