Non Michaelis-Menten Kinetics Flashcards
What particularly about the xenobiotic metabolizing enzyme do we focus on in Biochemical Pharmacology?
‘’Unusual’’ in vitro kinetic behaviour
What are the two main members under xenobiotic metabolizing enzymes?
Cytochrome P450 (CYP) enzyme system and UDP-glucuronosyl transferase (UGT) enzymes
What is autoactivation?
It results in an initial lag in the rate-substrate concentration profile generating a sigmoidal curve
What is autoinhibition?
It is characterized by a convex curve due to Vmax not being maintained at high substrate concentrations
What is the problem with the Michaelis-Menten model?
There are particular kinetic features for several CYP and UGT substrates that the M-M model cannot explain
What is cooperative binding?
Cooperative binding is when one substrate binds to make the next substrates easier to bind
What kind of enzyme model is expected from a Non-M-M kinetics?
Enzyme models with multiple sites showing cooperative binding for the drug substrate
Define allosterism
Allosterism is “binding at an alternative site
True or False: Allosterism in vivo exists
False: allosterism in vivo does not exist, but it does in an in vitro setup where it can either be co-operative or inhibitory
What is S50 and what is it analogous to?
S50 the 50% of the Vmax and it is analogous to the Km from the Michaelis-Menten equation
DOUBKE CHECK: What is n referring to in the Hill Model equation?
n is the unique Hill coefficient
Describe what 1B is by uncompetitive inhibition
v = Vmax/(1+(Km/S)+(S/Ksi))
Describe a substrate inhibition curve
Substrate inhibition curves are concentration-dependent: low concentration looks like M-M and in higher substrate concentration, the curves go “off piste”
What are Ks and Kp?
Ks is the substrate dissociation constant and Kp is the catalytic rate constant
What happens when α < 1?
The binding affinity for the second substrate molecule is increased (co-operative binding), which enhances the overall product formation rate and results in autoactivation
What is the change in Kp by the factor β?
It is when both substrate sites are occupied. This change is an alternative mechanism for autoactivation
What happens when β > 1?
The overall rate of the reaction is increased (autoactivation)
What happens when β < 1?
The overall rate is decreased (autoinhibition)
What can the model/kinetic scheme for substrate interactions at two separate binding sites be used for?
The model can be used to describe data from substrates showing both sigmoidicity/co-operation and substrate inhibition
True or False: there are no direct relationships between parameters from the model (model/kinetic scheme for substrate interactions at two separate binding sites) and the Hill coefficient, n, or the substrate inhibition, Ksi
True
How can a positive cooperative effect or sigmoidicity be observed?
When a value of α < 1 and β > 1
Which two conditions can observe a negative cooperative effect?
a. value of α > 1, resulting in a biphasic kinetic profile
b. value of α < 1, resulting in a biphasic kinetic profile
c. value of β > 1, causing substrate inhibition
d. value of β < 1, causing substrating inhibition
a. value of α > 1, resulting in a biphasic kinetic profile
and
d. value of β < 1, causing substrating inhibition
What problem do these models have that you need to be aware of?
The models do not distinguish between the simultaneous binding of multiple molecules within a single active site and the binding of two molecules at two distinct sites
How does in vitro-in vivo scaling strategy work?
This strategy is used to describe fully the in vitro data, and then abstract something useful for extrapolation to real human or animal drug research
Why is it important to comprehensively characterize in vitro systems?
To know and understand the limits that may extend beyond those observed in vivo
Why are substrate and product inhibition unlikely to be of consequence in vivo?
Due to the high concentrations required and because clearance is constantly occuring
What issue arises from forcing Michaelis-Menten kinetics in in vivo and/or in vitro setups?
Sigmoidal or inhibitory kinetics that may or may not be functions of the test tube system are not recognized and accounted for, which can affect what will happen in the human body due to inappropriately analyzed data
What is the minimum value for H in a CLmax equation and what does it correspond to?
Minimum of 0.5 H value that corresponds to n = 2 substrates binding
What are some of the processes that would lower the [Substrate] available to the enzyme relative to the concentration calculated, after the addition of a particular [Substrate] to the test tube?
- Substantial substrate depletion
- Saturable futile binding of substrate or metabolites (products) within the incubation matrix
- Saturable cellular efflux processes - in a whole cell culture system, transporters, especially ABC transporters may be pumping drug or product/metabolite out
- Poorly defined limit of analytical quantitation (concerns with the instrument used)
- Aqueous solubility limitations (precipitation of drug or metabolites)
What is the impact on processes that involve saturable events?
It would result to concentration-dependence (maximal at the low concentrations and tapering off to no effect at high concentrations)
What is the ideal turnover of substrate?
10% or less (<10%) to get sigmoidicity and to comply with initial rate conditions and prevent product inhibition
What is the role of active transporter systems?
To detect any inconsistency between cellular and media concentrations of drug substrates
If cellular efflux is significantly different than the Km for metabolism, sigmoidicity could ______
arise
What are some of the consequences of ignoring sigmoids?
- Problems with first in human or first in animal drug trials (oversight)
- Underprediction of CLint (calculated drug dose is sub-therapeutic)
- Underestimation of Vmac for substrate inhibition will occur by ignoring the high concentration data points and forcing a M-M model through the remaining lower [substrate] data (Km is poorly estimated)
- Bad choice of model to estimate clearance gives poor quality data points (not rich enough)
What is the problem with many Pharma/Biotech investigators?
They only work with one substrate concentration and asses inhibitory potential by the concentration responsible for a 50% inhibition value (I50)
How is the interpretation of the nonhyperbolic type behavior complicated?
Effects are not consistently seen with all substrates for the same enzyme as they vary by substrate or drug
