Enzyme Kinetics (Module 1 Lecture 6) Flashcards
How can proteases be inhibited and what is a benefit of protease inhibition?
- Protease inhibitors are specific.
- Trypsin, for example, is inhibited by a trypsin inhibitor (also produced in the pancreas) which is a competitive inhibitor that binds tightly to the substrate.
- Alpha-1 antitrypsin is a plasma protein that protects the lungs from damage caused by elastase.
Compare and contrast between competitive and noncompetitive inhibition.
- Competitive inhibitors don’t affect Vmax but increase Km (the y-intercept on a graph of 1/V as the y-axis and 1/S-concentration as x-axis will be the same as the line of no inhibitor present but the slope will be larger).
- Noncompetitive inhibitors decrease Vmax but don’t affect Km (the y-intercept will be higher and the slope will be higher).
Give an example of irreversible and reversible inhibitors.
- Ibuprofen is a reversible inhibitor because it binds to the active site without covalently attaching to anything (the same thing goes for some other cyclooxygenase inhibitors).
- Aspirin is an irreversible inhibitor of cyclooxygenase (aka prostaglandin synthase) that covalently transfers an acetyl group to Ser-530 on the enzyme.
What are two reasons a graph of concentration vs. time may not be linear in enzyme kinetics?
- Product inhibition
2. Enzyme instability
What are four things that affect the rate of an enzymatic reaction?
- pH (most mammalian enzymes are active at pH ~ 7 but lysosomal and digestive enzymes like pepsin require very acidic pH’s).
- Temperature (rates increase about two times per 10 degree Celsius increase in temperature but the problem is enzymes denature at higher temperatures, which will kill activity).
- Enzyme concentration
- Substrate concentration
When does competitive inhibition work best?
At low substrate concentrations.
How can competitive inhibition be fully overcome?
By sufficiently increasing substrate concentrations.
What are two ways a noncompetitive inhibitor can bind?
- To the enzyme without a substrate attached (EI complex)
2. To the enzyme with the substrate attached (ESI)
What are two types of enzyme inhibition?
- Reversible (either competitive or noncompetitive)
- Irreversible (either by binding tightly to the enzyme, by modifying amino acid side chains using irreversible chemical reactions, or by denaturing the protein)
What is one example of a toxin that acts as an enzyme inhibitor?
Organofluorophosphates are toxic nerve gases that inhibit acetylcholinesterase.
What are isozymes and why do they exist?
- Also called isoenzymes, they are two different enzymes that catalyze the same reaction.
- Isozymes help bodies fine tune things like metabolism.
Give an example of two isozymes that catalyze the same chemical reaction.
- Glucokinase and hexokinase both catalyze the storage of excess glucose in the form of glycogen and convert excess glucose to fat.
- Hexokinase works when glucose concentrations are low and livers have glucokinases ready for when blood sugar rises.
How can isozymes be distinguished from one another?
By using properties like electrophoretic motility.
What are the isozymes of lactate dehydrogenase?
There are five different LDH tetramers:
- H4 = heart, kidney, red blood cells, also used by brain and white blood cells.
- H3M = found in heart and kidneys but mainly used in red blood cells and. brain, also slightly used by white blood cells.
- H2M2 = contained in brain, leukocytes, and also in kidneys.
- HM3 = found in leukocytes.
- M4 = found in muscle and liver.
Contrast the isozymes of LDH and their changes during development.
- LDH-1 (H4) needs highly aerobic conditions and therefore is in its major form in the adult heart.
- LDH-5 (M4) works best with lower oxygen concentrations and is in it’s major form in the fetal heart.
