Enzyme Regulation Flashcards
Two general ways to handle when an enzyme has more than one substrate
Ternary Complex and Ping Pong
Ternary complex
Can either be random order binding of enzyme to either substrate, or an ordered way of binding (ie substrate one first then substrate 2, etc)
Ping Pong
Ex: Substrate 1 enters, then product one, then substrate 2, etc.
Do enzymes with two or more substrates have the same Km for each substrate?
No, there will be a different Km for each substrate
Example of Ping Pong
hexokinase binding to glucose and ATP
The two main classes of molecular inhibition
Reversible and irreversible
Inhibitor competes with the active site and there is no catalysis. Forms an EI complex
Competitive inhibition
Inhibitor binds to a spot other than the active site which renders the enzyme inactive. Forms an ESI complex
Uncompetitive inhibition
Inhibitor binds in spot other than the active site. Can bind either the E or ES complex but has a bias toward one. Is a true mix of the first two mechanisms
Mixed inhibition
Special case of mixed inhibition where alpha = alpha’. Can bind to E or ES complex
Non-competitive inhibition
Vmax stays the same (unchanged) and apparent Km increases
Competitive inhibition
Vmax decreases and apparent Km decreases
Uncompetitive inhibition
Vmax decreases and apparent Km stays the same
Non-competitive inhibition (mixed)
Can wash out effects of inhibitor by increasing the substrate concentration
Competitive inhibition
Can’t wash out effects of inhibitor by increasing the substrate concentration
Uncompetitive inhibitor and Noncompetitive inhibitor
Why can’t we wash out the effects of the uncompetitive and noncompetitive inhibitor with substrate?
Inhibitors are not bound to the active site, so substrate is already bound where it needs to be. Inhibitor just stops reaction from occurring.
Forms covalent bonds with the enzyme and are permanently bonded to the active site. Called suicide inhibitors
Irreversible inhibitors
Examples of irreversible inhibition
Penicillin with penicillin binding proteins
Why is penicillin so impressive?
The target enzyme of penicillin (transpeptidase) does not live in our bodies, it lives in bacteria. Because of this, when penicillin binds to bacteria to stop their function, our cells and enzymes stay the same so we are unlikely to have any side effects.
Type of inhibition when the end product of a pathway is the molecule that shuts that pathway down.
Feedback inhibition
Why is feedback inhibition important?
It prevents production of intermediates that the body won’t use, or intermediates that already exist in too high of quantities.
Regulatory enzymes that have 2 different subunits. One that has an active site and one that has the regulatory site.
Allostery
When binding of a molecule on one part of a protein affects the shape and behavior of the protein
Allosteric modulation
Enzymes that have a modulator that is different from the substrate. It binds outside of the active site
Heterotropic enzymes
When the substrate is the modulator and active site is the regulatory site
Homotropic enzymes
Do allosteric enzymes follow the M-M rules?
No. Change in rate caused by modulator creates a sigmoidal curve. No true Km
Homotropic enzymes have sigmoidal curves. What does that suggest?
Cooperativity
For heterotrophic enzymes, which regulator could lower Km but not alter the Vmax? (Opposite is ANOTHER possibility)
Positive regulator
For heterotrophic enzymes, which regulator could raise Km but not alter Vmax? (Opposite is ANOTHER possibility)
Negative regulator
Examples of reversible covalent modifications
Methylation, ADP-ribosylation, Acetylation, Adrenylylation, Myristoylation, Prenylation, Ubiquitination, Phosphorylation
Enzymes that add a phosphoryl group to a protein (add to Tyr, Ser, Thr)
Kinase
Enzymes that remove a phosphoryl group from an enzyme
Phosphatase
What does adding a phosphate group do to a protein?
Can cause it to be a disrupting, negatively charged “blob”, can block a binding site, can change the entire shape of the protein
Stored form of glucose kept largely in muscle and liver. Mobilized when urgent need for glucose oxidation or when there is low nutrition. Way we store energy in our bodies, “ball of energy”
Glycogen
Glucose-1-phosphate is turned into Glucose-6-phosphate which is used to enter glycolysis
Glycogen phosphorylase kinase
A protease that is activated by proteolytic cleavage breaks down protein in the gut
Trypsin
Pre-cursosr (pre-cleavage) trypsin in called what?
Trypsinogen
Trypsinogen is a zymogen. What does that mean?
It is the inactive pre-cursor. Needs to be cleaved to form the active enzyme
