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Flashcards in USMLE Gen Pharm Deck (426)
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1

Km: Definition

Km = Substrate at 0.5*Vmax
Km reflects the affinity of the enzyme for its substrate

2

Vmax indicates what?

Vmax is directly proportional to the enzyme concentration.

3

Relationship between Km and affinity

–The lower the Km, the higher the affinity
–Smaller Km means enzyme is saturated earlier, which means that small amounts of substrate are picked up by the enzyme.

4

Reading an inverse curve: Y–intercept equals ?

1/Vmax
The higher the Y–intercept the lower the Vmax

5

Reading an inverse curve: X–intercept equals ?

(1/–Km)
The further to the right the x–intercept, the greater the Km

6

Reading an inverse curve: Slope equals ?

Km/Vmax

7

Reading an inverse curve: Effect of a competitive inhibitor

X–intercept farther to the right, meaning Km is greater, because you need more substrate to get the same effect as the competitive inhibitor is hogging the enzyme.

The y–intercept is the same, meaning Vmax hasn't changed, because there isn't any more enzyme.


The slope is greater, because Km has increased while Vmax has stayed the same.

8

Reading an inverse curve: Effect of a noncompetitive inhibitor

The x–intercept is the same, meaning Km is the same, because the affinity for the enzyme hasn't changed, there's just less of it.


The y intercept has increased, meaning Vmax has decreased, because enzyme has been inactivated by the noncompetitive inhibitor

The slope is greater, because Vmax has decreased while Km has stayed the same.

9

Competitive inhibitor: Resemble substrate

Yes

10

Competitive inhibitor: Overcome by increased substrate?

Yes

11

Competitive inhibitor: Binds active site?

Yes

12

Competitive inhibitor: Effect on Vmax

Unchanged. The amount of enzyme has not changed.

13

Competitive inhibitor: Effect on Km

Increased. A lot more substrate needs to be available to seize the active sites.

14

Noncompetitive inhibitor: Resemble substrate?

No

15

Noncompetitive inhibitor: Overcome by increased substrate?

No

16

Noncompetitive inhibitor: Binds active site?

No

17

Noncompetitive inhibitor: Effect on Vmax

Decreased. Takes the enzyme out.

18

Noncompetitive inhibitor: Effect on Km

Unchanged. Does not change the affinity for the enzyme.

19

Volume of distribution: Abbreviation

Vd

20

Vd: Stands for what?

Volume of distribution

21

Volume of distribution: definition

Vd = (amount of drug in the body)/(plasma drug concentration)

22

Volume of distribution: What alters it?

Liver and kidney disease

23

Where are drugs with a low Vd distributed?

plasma

24

Where are drugs with a medium Vd distributed?

extracellular space

25

Where are drugs with a high Vd distributed?

tissues

26

Clearance: definition

(rate of elimination of drug)/(plasma drug concentration)

=Vd x Ke where Ke=elimination constant

27

Half life: definition

The time required to change the amount of drug in the body by 1/2 during elimination (or during a constant infusion).

28

What percentage of steady state is a drug at after: 1 half life

50%

29

What percentage of steady state is a drug at after: 2 half lives

75%

30

What percentage of steady state is a drug at after: 3 half lives

87.50%