regulation of protein function Flashcards
why are proteins regulated?
to ensure:
maintenance of cellular homeostasis
responsiveness to the environment
efficiency
what are the major ways in which enzyme activity can be regulated?
- changes in substrate concentration
- binding of small effector molecules
- reversible covalent modifications
- binding of regulatory proteins
- proteolytic activation
- controlling the amount of enzyme present
how can controlling substrate concentration regulate protein function?
- affects rate
- particularly important when [S] is less than Km, when rate is linearly dependent on [S]
what are isoenzymes?
enzymes that catalyse the same reaction but have different amino acid sequences
what are the roles of isoenzymes?
- can have different metabolic uses in different organs
- can have different locations and metabolic roles in the same cell
- can have different roles at stages of development
- can have different responses to allosteric regulators
give an example of an isoenzyme
lactate dehydrogenase
- main types: H4 + M4
- H4 is found mainly in the heart, has low Km for lactate and favours lactate oxidation
- M4 is found in skeletal muscle, has low Km for pyruvate and favours pyruvate reduction
what is allosteric regulation?
regulation mediated by interactions of a modulator at a regulatory site away from the active or binding site. binding of a modulator causes a conformational change
what is a homotropic modulator?
an allosteric modulator which is the same as the substrate
what is a heterotropic modulator?
an allosteric modulator which is different from the substrate
do allosterically modulated enzymes obey Michaelis-Menten kinetics?
no
give an example of allosteric regulation
co-operative oxygen binding in haemoglobin
haemoglobin undergoes a structural change on binding Hb, oxygen binding promotes stabilisation of high affinity R state
what do allosteric activators do?
increase the proportion of enzyme in the R state
what do allosteric inhibitors do?
increase the proportion of enzyme in the T state
when are covalent modifications made?
post-translationally
how do protein kinases cause covalent modification?
protein kinases transfer their terminal phosphate from ATP to the OH group of Ser/Tyr/Thre
how do protein phosphatases cause covalent modification?
catalyse the hydrolytic removal of phosphoryl groups from proteins
why is phosphorylation an effective covalent modification for regulation of protein?
- large free energy permits changes in conformation of phosphorylated protein
- addition of 2 negative charges disrupts existing electrostatic interactions
- phosphoryl group can form 3 new H bonds, allowing specific interactions with hydrogen bond donors
- permits amplification cascades
explain how phosphorylation by kinases results in signal amplification?
activation of enzymes by other enzymes increases number of affected molecules - allowing signal amplification of several orders of magnitude in a few ms
how is PKA an example of phosphorylation regulation?
in the absence of cAMP a pseudosubstrate region blocks activity of PKA
PKA consists of regulatory subunit and catalytic subunit
when cAMP binds to R subunit, it dissociates, releasing the C subunit which is catalytically active
what is the concerted model for allostery?
one substrate molecule causes complete change in R state
what is the sequential model for allostery?
substrate binding to one subunit will only affect adjacent subunits
why is CTP an ATCase inhibitor?
CTP favours the R state, stabilising the inactive state
what is the difference in response for allosteric enzymes vs those which obey Michaelis-Menten kinetics?
allosteric enzymes can respond to small changes in [S] compared to those which obey michaelis-menten
how do adaptor proteins spatially restrict signals in the cell?
a-kinase anchoring proteins have different domains specific for PKA, PP2A, the target protein and specific regions of the cell membrane
