lecture 4 Flashcards
steps of glycogen breakdown
broken down by phosphorylase to glucose-1-p to glucose-6-p to pyruvate
steps of production of glycogen
UDP-glucose to glycogen by glycogen synthase
OR
glucose to g-6-p to g-1-p to glycogen by phosophorylase
what is the role of glycogen phosphorylase in muscle
to provide ATP for muscle contraction
it is activated when ATP is in short supply, a muscle contracts and in a critical situation
what is generated by glycogen breakdown for glycolysis
g-6-p
how is glycogen phosphorylase allosterically regulated
glycogen phosphorlyase is activated by AMP
activation by amp is antagonised by atp and g-6-p
amp, atp and g6p dont bind the same site
allosteric regulation due to non covalent binding of a small molecule that binds to a site distinct from the active site
g6p is derived from the breakdown of glycogen and so is atp
g6p and atp are both classical feedback inhibitors of glycogen breakdown
what is the significance of phosphorylase activation by amp
amp is produced in many enzymatic reactions
major source is catalysed by adenylate kinase
it is a reversible reaction that uses very little free energy and it maintained close to equilibrium in most cells
if adp:atp ratio rises 2 fold, amp:atp rises 4 fold - effects of amp are antagonised by atp, phosphorylase is activated by increased amp:atp - a signal of low cellular energy status
glycogen is broken down to replenish atp when it is depleted
what does allosteric regulation maintain
homeostasis
how is allosteric mechanisms switched off
reverse the on function
constrained by the equilibrium constant for association:dissociation reaction
what are molecular switch mechanisms needed for
to cause a large change in signal output in response to a small change in signal input (large increase of AMP is needed to go from off to on)
molecular switched on and off pathways are different and usually involve covalent mods of proteins
adv of allosteric regulation
ligand binding site due to simple non covalent interaction = no enzyme is required
no energy directly consumed by association and dissociation of ligand
disadv of allosteric regulation
large changes in ligand conc required to achieve large effects
allosteric reg better for homeostasis than for making changes
adv of covalent modifications
interconversion reactions have high energy, must be catalysed by enzymes
for protein kinase reactions, equilibrium is driven by high cellular ratio of atp:adp
for protein phosphate reactions, equilibrium will always lie in favour of dephosphorylation - it is driven by high cellular water conc
small adjustments in relative activities of kinae and phosphate enzymes allow achievement of any desired ratio of protein:phosphoprotein from 0 to 1
protein phosphorylation is therefore good for switching cells from one state to another
disadv of covalent modifications
price paid for control of atp is consumed in the phosphorylation cycle
2 forms of phosphorylase
how are they two types found
b - allosterically activated by amp
a - doesnt require amp
a - phosphorylated at a single serine residue near n terminus
b - in dephosphorylated form
covalent mods of glycogen phosphorylase
phosphorylation is catalysed by a protein kinase ‘phosphorylase kinase’ which is activated by low calcium conc
muscle contractions triggered when stimulation of motor nerves trigger calcium release from sarcoplasmic reticulum of cell
glycogen breakdown is coupled with muscle contraction
covalent mods of phosphorylase kinase
phosphorylase kinase is phosphorylated by cyclic AMP - dependent protein kinase, increasing its maximal activity and calcium sensitivity
cAMP increased in the muscle by hormone adrenaline is released during exercise or stress
more rapid and complete conversion of phosphorylase to amp independent ‘a’ when an organism is under stress
hormonal mechanism superimposes on the nervous mechanis, while both over ride allosteric mechanism
g protein switch mechanism
conformational change in gtp binding or g protein
a signal activates the gtp/gdp exchange factor
causes a switch of g protein to active gtp bound conformation
active g protein binds and activates downstream effector protein such as cAMP
in absence of continuing stimulation, g protein reverts back to gdp inactive form
gtp hydrolysis is catalysed by g protein and promoted by gtpase activator protein
on and off are different so molecular switch mechanism criteria is fulfilled
how is protein phosphorylation a central mechanism
protein kinases often exist in sequences in which one kinase phosphorylates and activates another
as each kinases acts catalytically the signal is amplified
cascades often mediate effects of extracellular signalling molecules or first messengers
kinase casscades often converge and diverge from signalling networks
responses to extra and intra cellular signals can be integrated
dephosphorylation by protein phosphates may also be regulated
other types of covalent modification
phosphorylation, ubiquitination, acetylation (all reversible
what type of covalent mods are irreversible
those paused by pathogenic bacteria
bacteria causing cholera and pertussis produce protein toxins that enter the host cells and catalyse adp ribosylation of host proteins
cholera is acquired by drinking infected water - its toxin modifies a g protein that controls cAMP production in cells lining airways, causes fluid secretion and severe coughing spreading the infection back into the environment
toxins are v potent as the act catalytically and because host cells have no enzyme to reverse ADP ridosylation
blood clotting switch mechanism
reversible switch mechanisms require energy and therefore need the presence of active donors of the modifying group such as atp or nad
outside cells active donors are not present so irreversible modifications are used instead
blood clotting uses cascade of proteinases that activate each other by proteolysis
each clotting factor cleaves an inactive precursor off of its downstream target and activates it
extrinsic clotting pathways triggered when factor 7 forms a complex with tissue factors released from the damaged cells
fibrinogen is soluble but fibrin is insoluble and it aggregates to start the blood clot
proteolysis is irreversible and once used up new clotting factors must be synthesised
