Regulation of enzyme activity Flashcards
(33 cards)
What is an isoenzyme?
-Same enzyme with different kinetic properties, produced by alternate splicing of the same gene which preoduces a different a’a sequence
How does substrate/product concentration control enzyme activity?
-[Substrate] affects the rate of activity of the enzyme
Give an example of isoenzymes and why these are clinically important
- Hexokinase and Glucokinase
- Glucokinase has high Km; Hexokinase has low Km
- Clinically important because glucokinase predominantly found in liver and only reaches effective enzyme saturation at high concentrations of glucose, i.e. after a meal
What type of enzymes does allosteric regulation occur with?
-Multicomplex enzymes which have more than one active site
Why do enzymes which are allosterically regulated have sigmoidal rate curves?
-Positive cooperativity -> binding one active site increases the affinity of the other active sites
What are the two conformations of allosterically controlled enzymes?
- R and T forms
- Allosteric inhibitors stabilise the low affinity T form
- Allosteric activators stabilise the high affinity R form
How are some enzymes controlled by covalent modification?
- Covalent attachment of various groups to the enzyme which results in activation or inhibtion of the enzyme
- Eg phosphoryl, adenyl, acetly etc
Which is the most important form of covalent modification?
-Phosphorylation which usually activates an enzyme and is catalysed by a kinase enzyme
What is proteolytic activation of enzymes?
-Proteolytic cleavage of part of the polypeptide
What is the most frequent function of proteolytic activation?
-Activate zymogens
What are zymogens?
-Inactive precursor molecules eg pepsinogen -> pepsin, Insulin precursors, blood clotting factors, proteolytic enzymes
Why are some proteins synthesised in an inactive form?
- Ensures safe transport to site of action
- Only want certain enzymes to be active under certain conditions, eg insulin and high blood glucose
What is meant by a proteolytic cascade?
-proteolytic activation of one enzyme will lead to proteolytic activation of a series of other enzymes
Give an example of a proteolytic cascade in the pancreas
-Master regulator Enteropepsidase proteolytically cleaves trypsin which in turn proteolytically activates a series of other enzymes, including itself
How is chymotrypsinogen activated?
- Proteolytic cleavage by trypsin leaves a partially active chymotrypsin and pi-chymotrypsin
- pi-chymotrypsin activates itself
- leads to cleavage of the partially active chymotrypsin
- the three segments are bonded together producing fully active chemotrypsin
How is proteolytic activation regulated?
- Regulated by endogenous inhibitors binding to protease stoppong cleavage
- One inhibitor can inhibit a range of proteases
Give an example of a clinical condition caused by a deficiency in protease inhibitors
-emphysema -> elastase not regulated causing destruction of alveoli
Name the long-term regulation of enzyme activity
- change in transcription/translation
- Change in the rate of protein degredation -> upregulation of ubiquitin proteosome pathway -> unbiquitin regulates protein breakdown by tagging proteins to be destroyed.
What activates the intrinsic pathway of the clotting cascade?
-Damage to endothelial lining promotes binding of factor XI
What is the blood clotting cascade a proteolytic activation cascade?
-Proteolytic activation of one factor ptoteolytically activates many others
What causes the extrinsic pathway of the clotting cascade to begin?
-Membrane damage exposes the extracellular domain of factor III which causes autocatalytic activation of factor VII
At what point do the two cascade converge?
-On factor X
What are the important cofactors?
-V and VIII
How are blood clotting factors normally present in the blood and why?
-In very low concentrations and in zymogen form to prevent accidental activation
