lecture 12 - control of enzyme activity Flashcards
What are enzyme inhibitors?
Compounds that bind to enzymes and reduce their activity
What are the 2 classes of enzyme inhibitors?
Reversible and irreversible
What are irreversible inhibitors?
Inhibitors that permanently bind covalently to enzymes, usually in the active site
What type of bonds are formed between irreversible inhibitors and amino acids in the active site of enzymes?
Covalent bonds
What are the 3 classes of reversible inhibitor?
Competitive, non-competitive, mixed
How do reversible inhibitors bind to enzymes?
Non-covalently
What is a competitive inhibitor?
A reversible inhibitor that’ competes directly with the substrate for the active site, creating two possible mutually exclusive binding events
How does a Vo vs [S] graph change when a competitive inhibitor is added?
No change in Vmax, because infinite [S] eventually outcompetes the inhibitor. However, the curve is flatter and moved to the right because a higher [S] is needed to reach Vmax - therefore higher Km
How is the Km value affected when a competitive inhibitor is added?
Increased Km - because a greater [S] is needed to reach Vmax/2
How is the Vmax value affected when a competitive inhibitor is added?
Unaffected
How does a Lineweaver-Burke Plot change when a competitive inhibitor is added?
Steeper line, same y intercept, higher x-intercept (less negative, smaller number)
What is a non-competitive inhibitor?
An inhibitor that binds a different site to the substrate (not the active site), so has no affect on the binding of S (in the case of pure non-competitive inhibition) . However, binding of I changes the structure of the active site such that transition state stabilisation of S is no longer optimal
How does a Vo vs [S] change when a non-competitive inhibitor is added?
Vmax is less, so curve reaches the asymptote at a lower point. Km remains the same because the binding affinity is the same
How is the Km value affected when a non-competitive inhibitor is added?
Km value is unchanged because the binding affinity of the substrate is not affected by a non-competitive inhibitor
How is the Vmax value affected when a non-competitive inhibitor is added?
Decreases because the product is made more slowly as transition state stabilisation is impaired by the inhibitor
How does a Lineweaver-Burke Plot change when a non-competitive inhibitor is added?
Steeper line, same x intercept, higher y-intercept
How does mixed inhibition change Vmax?
Decreases Vmax
How does mixed inhibition change Km?
Increases Km
How does mixed inhibition alter a Lineweaver-Burke plot?
Slope, and both x- and y-intercepts change
What is enzyme regulation?
Turning enzymes ‘on’ or ‘off’
What is the key function of feedback and feed forward enzyme regulation?
Avoids making unnecessary metabolic intermediates
What chemical helps to active the glycogen phosphorylase enzyme?
AMP
How does AMP activate the glycogen phosphorylase enzyme?
Binds to an allosteric binding site (into active site) and creates a conformational change that activates it. Energy is required
Is AMP activation of glycogen phosphorylase feedback or feed forward?
Feedforward
How is glucose-6-phosphate used in a feedback mechanism in glycogen phosphorylase regulation?
glucose-6-p can bind to the glycogen phosphorylase enzyme in the same site as AMP, causing feedback inhibition, and preventing further ATP production so that glycogen can be stored
How does caffeine and purines regulate glycogen phosphorylase?
Via feedback mechanism - they bind allosterically to inhibit the enzyme
How do cellular signals regulate glycogen phosphorylase?
Phosphorylase kinase creates a conformational change that promotes enzyme activity
What are allosteric enzymes?
Enzymes that contain a region where small regulatory molecules (effectors) can bind and thereby alter the enzyme’s ability to bind substrate by creating a conformational change
What is the shape of a Vo vs [S] plot when an allosteric enzyme is added?
sigmoidal, rather than hyperbolic. S shape in the physiological [S] range
What are the 5 methods of enzyme regulation?
Covalent modification, allosteric effects, proteolytic cleavage, gene expression, enzyme degradation
How do non-competitive inhibitors lower “enzyme concentration”?
They lower the number of enzymes that are active, essentially reducing the number of enzymes that can undergo susbtrate reactions. As Vmax ultimately depends on enzyme concentration, Vmax is reduced
