Flashcards in L5 - regulation of protein activity Deck (33):
How can enzyme activity be controlled by transcription/translation?
1. Extracellular signalling -> transcription factors
3. mRNA degradation
Once an enzyme has been synthesised, how can its activity be controlled?
1. Protein degradation (ubquitin, proteasome)
2. Sequester in subcellular organelle (ER)
3. Substrate concentration
4. Allosteric activators and inhibitors
5. Covalent modification (phosphorylation/de-P)
6. Enzyme combines with regulatory protein
List the two major short term methods of regulating protein activity
1. Substrate and product concentration
2. Change in enzyme conformation:
(i) Allosteric regulation
(ii) Covalent modification
(iii) Proteolytic cleavage
List the two major long term methods of regulating protein activity
1. Change in rate of protein synthesis
2. Change in rate of protein degradation
What product inhibition occurs with hexokinase?
Glucose-6-phosphate (product) inhibits hexokinase activity (but not glucokinase)
What are isoenzymes?
Different forms of the same enzyme that have different kinetic properties (Km, Vmax) e.g. hexokinase and glucokinase
Apart from product and substrate availability, availability of what can sometimes affect enzyme activity?
Coenzymes e.g. NAD/NADH
Define a co-enzyme
a non-protein compound that is necessary for the functioning of an enzyme
What is product inhibition?
Accumulation of the product of a reaction inhibits the forward reaction
What is the relationship between rate and substrate concentration of allosteric enzymes and why?
Sigmoidal - due to the two conformations they exist in R state (high afinity) and T state (low afinity). Binding of substrate to one subunit creates a conformational change that makes subsequent binding to other subunits progressively easier
What is the effect of allosteric activators?
Increase the proportion of enzyme in the R state
What is the effect of allosteric inhibitors?
Increase the proportion of enzyme in the T state
List some common covalent modifications of proteins
What are the cofactors of protein kinases?
What are the cofactors of protein phosphatases?
H2O (hydrolysis of phosphdiester bond)
What is the purpose of enzyme cascades?
Allow amplification of initial signal (activated enzymes) by several orders of magnitude within a few milliseconds. FAST RESPONSE!
How is the breakdown and synthesis of glycogen reciprocally regulated?
Cell signalling cascade leads to the activation of protein kinase A (PKA) which can concurrently lead to:
1. The activation of glcogen phosphorylase enzyme
2. The deactivation of glyocgen synthase enzyme
List 5 processes in which specific proteolysis activates enzymes?
1. Digestive enzymes - zymogens -> active enzyme
2. Some protein hormones (proinsulin-> insulin)
3. Blood clotting
4. Developmental processes - zymogens activated to contribute to tissue remodelling
5. Apoptosis - proteolytic enzymes activated
What enzyme proteolyically cleaves the zymogens of pancreatic proteases?
Trypsin (which itself is activated by the proteolytic cleavage of an enteropeptidase)
What is the name of the protein that inhibits trypsin activy and a range of proteases?
Alpha1- antitrypsin (deficiency of this protein causes elastase to destroy alveolar wall causing emphysema).
How is the rate of protein synthesis controlled?
Enzyme induction/ repression
What activates the intrinsic pathway of the blood clotting cascade?
Damaged endothelial lining of blood cells promotes binding of factor XII
What activates the extrinsic pathway of the blood clotting cascade?
Trauma causing vascular damage releases tissue factor (factor III)
What is the endpoint for both the intrinsic and extrinsic pathways?
Factor X activation which activates thrombin and therefore the formation of a fibrin clot
How do Gla domains help bring together clotting factors?
Damage to capillaries attract Ca2+ (Ca2+ binds to membrane of platelets) and brings all negatively charged clotting factors together in one place where they can activate each other. Therefore activation of these clotting factors only occurs next to the site of damage and therefore clots are localised to the sites of damage
How is the soft clot formed by interaction between fibrin. How is the hard clot formed?
Amide bonds formed between the side chains of lysine and glutamine residues in different monomer, catalysed by transglutaminase, stabilise the newly formed soft clot -> hard clot (protransglutaminase is activated by thromin to transglutaminase)
What is the cause of classic haemophillia?
Defect in factor VIII (a cofactor which stimulates factor IX in the clotting cascade). It is a part of a positive feedback mechanism and its activity is increased by proteolysis of thrombin and factor Xa (activation of factors further downstream in the blood clotting cascade)
What is thrombins role in feedback mechanisms of the blood clotting cascade?
Positive feedback - directly on clotting factors (allosteric activation) or indirectly by activating cofactors of clotting factors (proteolytic cleavage?)
How is the blood clotting process stopped?
1. Localisation of (pro)thrombin - e.g. dilution of clotting factors by blood flow and removal by liver
2. Digestion of proteases - e.g. by protein C
How is protein C activated?
Thrombin binding to endothelial receptor, thrombomodulin
Name an inhibitor of the blood clotting cascade and its associated cofactor
Antithrombin III (AT3) - enhanced by heparin binding (cofactor)
List the key control points of blood clotting
1. Inactive zymogens present at low concentrations
2. Proteolytic activation
3. Amplification of initial signal by cascade mechanism
4. Clustering of clotting factors at site of damage
5. Feedback activation by thrombin ensures continuation of clotting
6. Termination of clotting by multiple mechanisms
7. Clot breakdown controlled by proteolytic activation