Flashcards in Lecture 11 - Enzyme Regulation Deck (50):
Basic concept of zymogen
The active site is not formed correctly until there's a proteolytic cleavage at a specific activation site on the chain.
How is a zymogen converted into an active enzyme?
When triggered, proteolytic cleavage occurs rapidly, resulting in nearly instant activation of enzyme activity.
The zymogen form of thrombin
True or false: Thrombin is a serine protease.
Where is thrombin made?
On a membrane surface
How many proteolytic cleavages need to be made in order to activate thrombin?
Which cleavage activates thrombin that has been released from the membrane?
The peptide bond that allows isoleucine 16 to swing in and form a H-bond with aspartate 194 in order to put the serine 195 in the right position to be an active nucleophile
True or false: Activation of thrombin is reversible.
How many polypeptide chains does mature thrombin have and what kind of bond holds them together?
It has two chains (a light chain, residues 1-15, and a heavy chain, residues 16 to the end) and they are linked by a disulfide bond
True or false: The mature form of thrombin is water soluble.
What is the post-translational modification that is made to prothrombin that allows it to bind to the membrane?
y-carboxylation of particular glutamate residues on the N-terminus
Why does that modification allow prothrombin to bind to the membrane?
y-carboxy-glutamate has an extra negative charge allowing it to bind positively charged calcium ions
What does the binding of calcium ions allow prothrombin to do?
It exposes hydrophobic residues that can bind the membrane
What is the cofactor for the carboxylation of the N-terminus of prothrombin?
What do Vitamin K antagonists do?
They interfere with the carboxylation of prothrombin, therefore it doesn't bind to membranes well and it will not form active thrombin
Why does prothrombin need to bind a membrane surface?
The activating protease for thrombin exists on the membrane and prothrombin must be near this enzyme in order to be activated
What does the activating protease for thrombin do?
It releases thrombin from the membrane surface
What does thrombin catalyze?
The conversion of soluble fibrinogen to insoluble fibrin that will form a blood clot
How do protein inhibitors work?
They bind so tightly to a given enzyme that the enzyme-inhibitor complex as a whole is degraded
What is an example of an enzyme-inhibitor complex?
What is heparin and what does it do?
It is a commonly prescribed short acting anticoagulant that promotes antithrombin-thrombin complex formation
What is a common vitamin K antagonist?
Alpha-1-antitrypsin or alpha-1-antiproteinase
A protein that degrades elastin
A protein in lung tissue that gives it flexibility
What does smoking cause?
Oxidation of alpha-1-antitrypsin
What are the consequences of oxidized alpha-1-antitrypsin?
It does not effectively inhibit elastase, elastase cleaves elastin without regulation and this causes lung scarring and emphysema
What does protein phosphorylation or dephosphorylation do?
It modifies the charge of an amino acid (like serine, threonine, or tyrosine) residue and if that residue is in or near the active site, the activity of the enzyme may be changed (increased or decreased)
What are the different kinds of enzyme inhibitors?
Competitive, noncompetitive, irreversible, allosteric
Binding site only; Km in presence of inhibitor will increase (1/Km will decrease); sometimes the immediate product of a reaction can compete with the substrate
Do not interfere with substrate binding; does not change Km, but they will decrease vmax (1/vmax will increase); interfere with catalytic machinery
Vmax is proportional to...
the amount of enzyme present
Non-competitive inhibitors that bind enzymes and do not release them; this decreases the amount of enzyme present and decreases the vmax
True or false: Inhibitors showing a drop in vmax can be reversible or irreversible.
True or false: Changes in the substrate concentration do not change enzyme activity.
Usually contain multiple subunits; does not usually follow Michaelis-Menten kinetics; can define an "apparent" Km; usually have binding sites for affector molecules; often regulate a reaction pathway
True or false: Allosteric enzymes are very sensitive to substrate.
True or false: Allosteric enzymes are loosely regulated by substrate concentration.
Example of an allosteric enzyme
Phosphofructosekinase 1 (PFK1)
Allosteric enzymes are often where in a dedicated reaction pathway?
At the beginning
What is an allosteric activator of PFK1?
What is an allosteric inactivator of PFK1?
Why is PFK1 inhibited by high amounts of ATP?
High amounts of ATP signal that the cell is energy rich and glycolysis should not continue
Why is PFK1 activated by high amounts of AMP?
High amounts of AMP signal that the cell is energy poor and that glycolysis should continue
How many catalytic sites does PFK1 have?
What does the Ki mean?
It marks the affinity of a substrate for an inhibitor.
Allosteric activators drive enzymes to what form?
Allosteric inhibitors drive enzymes to what form?
Where do ATP and AMP bind PFK1?