Flashcards in "Molecular & Cellular Princ Med Enzymes as Biological Catalysts Patricia Kanepopp" GABY Deck (37):
Why do we say that enzymes play a central role in biology?
1. Carry out almost all chemistry required by living systems
2. Permit a wide range of chemical reactions in a narrow set of conditions
3. Allow rapid, efficient adjustments to environmental conditions
Medical Aspects of Enzymology
1. Diseases traceable to enzymatic defects
2. Use of enzyme activities in diagnosis
3. Drugs as enzyme inhibitors
4. Therapeutic administration of enzymes
5. Design of therapeutic enzymes and enzyme inhibitors
General Characteristics of Enzymes
1. Tremendous enhancement of reaction rate
2. Reactions occur under relatively mild conditions
3. Extreme specificity
4. Capacity for regulation
Why are enzyme kinetics important?
It serves as an important tool in understanding enzyme mechanism and manipulating enzyme function
Before Vmax is achieved, the initial rate of an enzymatic reaction depends on ___
After Vmax is achieved, the initial rate of an enzymatic reaction does not depend on ___
What happens to the enzyme-substrate complex (ES) after it is formed?
It can either dissociate into E+S
it can go on to form product
Must an ES complex form for the reaction to take place?
At low substrate concentrations, ___ limits how much ES can form, so v is ___ on [S]
At high substrate concentrations, ___ limits how much ES can form, so v is ___ of [S]
What do k1, k2 and k3 mean?
They are kinetic constants
k1 of the --> E+S=ES reaction
k2 of the --> ES=E+S reaction
k3 of the --> ES=E+P reaction
Is it possible to determine the kinetic constants relative values?
Yes, by making some assumptions
v=(Vmax)X[S]/([S] + Km)
Vmax = k3 [E]t
Km = (k2 + k3)/k1
Can Km and Vmax be determined experimentally?
You can estimate Km and Vmax from a v vs [S] plot, but they are more accurately determined from a linearized form of the Michaelis-Menten equation (Lineweaver-Burk plot)
What does the Km mean?
Km = [S] at which v = 1/2 Vmax
Km will always have units of ___
The catalytic rate of an enzyme is MOST SENSITIVE to [S] when ___
[S] << Km
Knowing the Km allows you to calculate what fraction of enzyme molecules are bound to substrate at any [S]
fES=(v/Vmax)= [S]/([S] + Km)
When k2>>k3, i.e. the substrate binds and dissociates many times before it goes onto form product...
Km is approximately the same as the dissociation constant (Kd) for the ES complex under these conditions
￼Km is approximately equal to Kd when ___
*Km = k2/k1 = Kd for ES complex
T or F: Enzymes with low vs. high Km are adapted to different physiological functions
Examples: hexokinase (low Km) & glucokinase (high Km)
What does the Vmax mean?
Vmax = maximal velocity achievable for a specific concentration of enzyme
*Note: Vmax will have units of velocity
What is kcat?
A constant that is independent of enzyme concentration
kcat = Vmax/ [E]t
*[E]t = total enzyme conc.
What does kcat/Km allow?
Direct comparison of enzyme efficiencies
Why does higher kcat/Km imply higher efficiency?
kcat--determines how quickly ES complex is used
Km--describes how much ES complex is available
Why can we determine [E]t from reaction rate under Vmax conditions?
Because under Vmax conditions (enzyme saturated with substrate) v is directly proportional to the concentration of enzyme present
Lineweaver-Burk plot equation
1/v = 1/Vmax + (Km/Vmax)(1/[S])
Irreversible enzyme inhibitors
Bind very tightly to an enzyme and inactivate an essential functional group on the enzyme
Reversible enzyme inhibitors
Bind reversibly to enzyme and give temporary inactivation while they are bound
What types of reversible enzyme inhibitors are there?
- compete with substrate for binding to active site
- inhibition can be overcome by increasing [S]
- Km appears to increase in presence of inhibitor; Vmax does not change
- bind to a site on the enzyme other than the active site
- inhibition cannot be overcome by increasing [S]
- Vmax appears to decrease in the presence of inhibitor; Km does not change
What does the Ki mean?
1. It quantifies the strength with which an inhibitor binds to an enzyme
2. It is the dissociation constant for the enzyme-inhibitor complex
T or F: Higher Ki implies weaker inhibition
A graph of 1/[S] (x axis) and 1/v (y axis) and several [I] (inhibitors) plotted, will intersect at the y axis if I is a ___ inhibitor
A graph of 1/[S] (x axis) and 1/v (y axis) and several [I] (inhibitors) plotted, will intersect at the x axis if I is a ___ inhibitor