Biochem Lecture 11 Flashcards
What is the Michaelis-Menten equation and explain what it represents graphically.
V0=(Vmax*[S]) / (Km+[S])
The graph represents the rate of product produced in relation to substrate concentration. It comes from many experiments with constant enzyme concentration but varying [S] and the initial reaction velocity is measured.
What is Km?
Constant value for each enzyme.
Represents the substrate concentration where V0 is half of Vmax.
It is a measure of binding affinity of the enzyme for the substrate. It is inversely proportional to the affinity of the enzyme for the substrate.
If an enzyme has a high affinity for substrate, will its Km be high or low?
It will be low. This means the graph will be steep at the beginning and reach its Vmax quickly because it likes to react with the substrate.
How is Km determined graphically on a Michaelis-Menten plot?
Find Vmax/2, and find the corresponding [S] value for that point on the graph. This concentration is Km.
How is Km determined on a Lineweaver-Burke plot?
-1/Km is the value on the x-axis where the line intersects the x-axis.
How is Vmax determined on a Lineweaver-Burke plot?
The intersection of the line with the y-axis is equal to 1/Vmax.
What is Vmax?
It is the point at which the enzymes are fully saturated with substrate. It is the fastest rate at which the reaction can occur, and any changes to [S] will have little effect on the rate.
Is Vmax a constant?
No, it is dependent upon the enzyme concentration. If the enzyme concentration is doubled, Vmax is also doubled.
Explain the significance of Km using the example of glucokinase and hexokinase.
Hexokinase is found in most tissues and a low Km or high affinity for glucose. This means that it reaches its Vmax quickly and changes in glucose concentration have little effect on the rate of glucose uptake in these tissues. This is how glucose is metabolized in the brain.
Glucokninase has a high Km value or low affinity for glucose and is found in the liver. This means that at low concentrations of glucose the liver processes glucose very slowly, but processes it faster as glucose concentration increases. This allows the liver to uptake glucose at times of hyperglycemia, and leave glucose alone during times of hypoglycemia.
What is the meaning of Kcat (turnover number)?
This is a measure of the efficiency of an enzyme. It has units of product per enzyme per second. It is calculated from the following equation:
Kcat=Vmax/[E]total
What are isoenzymes?
Two different enzymes that catalyze the same reaction. They differ in Vmax and Km values and regulatory functions and dependence on cofactors. They tend to be located in different places and different kinds of tissues to have varying effects to meet different needs.
What are the two types of reversible inhibitors?
Competitive and non-competitive
How do competitive inhibitors work and what is their effect on Km and Vmax?
Resemble the substrate
Bind to the active site
Vmax is unaffected because if enough substrate is added it will out-compete the inhibitor and reach its Vmax anyway.
Km is shifted to the right because it requires more substrate to reach the same speeds as before. Essentially the affinity of the enzymes for substrate has decreased.
How do non-competitive inhibitors work and what is their effect on Vmax and Km?
Do not resemble substrate
Bind to a site other than active site
Enzyme can still bind substrate, but it is inactive
Km is unchanged because the enzyme affinity for substrate is the same.
Vmax will change because less of the enzymes are actually able to carry out the reaction.
How do irreversible inhibitors work?
They bind to enzymes permanently through covalent bonds and disable the enzyme completely.
