Lecture 2 Strategies for metabolic control Flashcards
What is the difference between a constitutional enzyme and an inducible enzyme?
A constitution enzyme usually has a long lifespan and performs central functions within the cell. It needs to be present in constant concentrations and therefore the rate degradation is equal to the rate of synthesis.
An inducible enzyme is only synthesised by the cell in times of need and usually has a short lifespan.
What gives rise to the short lifespans of inducible enzymes?
It could be the that a particular amino acid sequence is particularly vulnerable to attack by preteolytic enzymes or ubiquitination which marks them as targets for specific proteases.
How are the concentrations of inducible enzymes regulated?
They are regulated by metabolites, hormones and growth factors. They are also regulated by the amount of expression of the genes responsible for their creation.
Explain how insulin acts as an up-regulator of glycolysis.
Insulin induces the enzymes glucokinase and phosphofructokinase in the liver. These enzymes are in the glycolytic pathway and therefore a higher concentration of these enzymes results in higher rates of glycolysis.
How can localisation of enzyme expression serve to differentiate the function of tissue types in terms of metabolism?
By only expressing enzymes that catalyse certain reaction within a tissue you can make sure that substrates are converted into the necessary products. An example is the fate of G-6-Pase which converts G-6-P into glucose. This enzyme is found in gluconeogenic tissues such as the liver and kidney but not glycolytic tissues such as brain muscle and fat. This means that all the G-6-P in these tissues will go on to enter glycolysis with none going on to form glucose.
What are isoenzymes?
Enzymes that have a different primary structure but catalyse the same reaction. They are usually products of different genes.
How can isoenzymes be used in the clinical diagnosis of disease?
Different isoenzymes will be expressed in different tissues. When these tissues are damaged, by testing which isoenzyme is present, we can know where the damage is located. For example, the enzyme that converts pyruvate into lactate is called lactate dehydrogenase and it exists as different isoenzymes in the heart and in the liver and skeletal muscle.
Describe the rectangular hyperbola shape of the Michaelis-Menten kinetics model for enzymes.
The shape is hyperbolic. It increases proportionately with substrate concentration at low levels of concentration. This rate of reaction plateaus to Vmax at which point the enzyme is saturated and any increase in substrate concentration can not increase the rate of reaction.
Explain how the differences in Km for glucokinase in the liver and Hexokinase 1 in the muscles allows for their different glucose metabolism rates?
Glucokinase in the liver has a high Km. This means that there needs to be an abundance of glucose before it rapidly converts it to G-6-P. Hexokinase 1 has a low Km, this means it will convert glucose to G-6-P even at low levels of glucose. These differences mean that the liver will only convert glucose to G-6-P after a meal when there is high blood glucose whereas the muscle will continuously use up the glucose present in the cell.
What are the differences in inhibition between glucokinase and hexokinase 1?
Hexokinase 1 is allosterically inhibited by G-6-P. This is an example of negative feedback and stops G-6-P from building up in the muscle cells.
Glucokinase is not inhibited by the product G-6-P but it is subject to allosteric control by other intermediary metabolites.
What is allosteric modulation and how can it activate or inhibit enzyme activity?
Allosteric modulation occurs when a modulator (positive or negative) binds to a site other than the the active site which brings about a conformational change of the enzyme. In the case of a positive modulator, this change will make the active site change in such a way that it catalyses the reaction better. The opposite is true for a negative modulator.
Describe the kinetics of allosteric regulation.
The enzymes do not follow a Michaelis-Menten curve. The shape is often sigmoidal. This represents the cooperative interactions between protein subunits.
Describe how reversible covalent modification of an enzyme can regulate its activity. Use PFK-2/FBPase-2 as an example.
The enzyme Phosphofructokinase-2 catalyses the reaction of F-6-P to F-2,6-BP. The enzyme fructose bisphosphatase-2 catalyses the reverse reaction. These enzymes differ only by a phosphate group. If the enzyme is phosphorylated, the kinase is inactive and the phosphatase is active. If the enzyme is dephosphorylated, the kinase is active and the phosphatase is inactive.
What is a zymogen?
It is a catalytically inactive precursor form of an enzyme. After cleaving a limited number of specific peptide bonds the enzyme will become the active form.
