regulation of metabolism Flashcards
why do we need strict control of some enzymes involved in metabolic processes
because To co-ordinate metabolic processes, it is vital
name some factors that may be involved in the regulation of biochemical pathways
Concentrations of substrates and products
Modifications
Endocrine signals
Other enzymes
General Mechanisms of Enzyme Regulation is done by what
the Michaelis-Menten Equation
Leonor Michaelis and Maud Menten proposed what
proposed that the enzyme reversibly combines with its substrate to form an ES complex that subsequently breaks down to product, regenerating free enzyme.
The Michaelis-Menten equation describes what
how reaction velocity varies with substrate concentration
explain some assumptions of Michaelis-Menten Equation
[S] is small is much greater than [E], so that the amount of substrate bound by the enzyme at any one time is small.
[ES] does not change with time as it is in ‘steady state’.
Only initial velocities are used in analysis of enzyme reactions as it is only at this time when the reaction is linear with time.
The Michaelis constant (Km) is characteristic for what and what does this reflect
for an enzyme and its substrate and reflects the affinity of the enzyme for that substrate.
Km is numerically equal to what
equal to [S] at which the reaction velocity is at ½Vmax.
Km does not vary with what of an enzyme
conc of an enzyme
explain small Km
A numerically small (low) Km reflects a high affinity of the enzyme for substrate because a low concentration of substrate is needed to half-saturate the enzyme i.e. to reach ½Vmax.
Enzymes like this are usually targeted for regulation.
explain high Km
A numerically large (high) Km reflects a low affinity for the substrate because a high concentration is required to half-saturate the enzyme.
Enzymes like this are not usually targeted for regulation.
what is rate of reaction proportional to
to the enzyme concentration at all [S].
what does it mean when [s] is much greater than Km in a reaction
the velocity is constant and equal to Vmax. The rate of reaction is then independent of [S] and is said to be zero order with respect to substrate concentration.
what does it mean When [S] is much less than Km in a reaction
the velocity of the reaction is roughly proportional to [S]. The rate is said to be first order with respect to substrate.
what are effectors
molecules that may bind non-covalently to an enzyme and inhibit its activity.
name and explain the 2 types of effector
Homotropic Effectors:
When the substrate is an effector
They are usually positive
Significance: Km is decreased several fold for a small increase in [S].
Heterotropic Effectors:
The effector is not the substrate
May have a stimulatory or inhibitory effect
Very rapid (instantaneous) form of regulation
explain Induction and Repression of Synthesis
Represents adaptive regulation whereby enzyme synthesis is either enhanced or decreased by certain physiological situations.
Slow (days) mechanism of regulation.
what is an example of induction and repression synthesis
glycogen metabolism
as Processes of glycogen synthesis and glycogen breakdown are reciprocally regulated allosterically, covalently, and by induction/repression of synthesis
what are the 2 key regulatory enzymes in glycogen metabolism
SYNTHESIS: Glycogen Synthase
BREAKDOWN: Glycogen Phosphorylase
in a fed state glucose in broken down to what
glycogen
activated by glycogen synthesis
inhibited by glycogen breakdown
in a fasted state glycogen in broken down into what
glucose
activated by glycogen breakdown
inhibited by glycogen synthesis
what causes allosteric regulation of glycogen metabolism and explain how
glucose -6 - phosphate
as Glucose 6-phosphate activates Glycogen Synthase and inhibits Glycogen Phosphorylase
what affect does AMP have on glycogen synthase
has no effect on Glycogen Synthase, but allosterically activates Glycogen phosphorylase – very important in exercising skeletal muscle
in glycogen metabolism when is AMP used and what does it generate
ATP is used when skeletal muscle contract , generating AMP
as glycogen produces glucose -1-phosphate by glycogen phosphorylase activated by AMP. this then becomes glucose -6-phosphate which is then absorbs by muscle by glycolysis and ATP
Adenosine Monophosphate (AMP)
explain ATP in terms of allosteric regulation of glycogen metabolism
↑↑ [ATP] means the energy status of the cell is very high, hence there is no requirement to breakdown Glycogen
ATP allosterically inhibits Glycogen phosphorylase (in converting glucose to 1-phosphate), but has no effect on Glycogen Synthase
(Its like the opposite of AMP as lots energy)
explain glucoses effect in allosteric regulation of glycogen metabolism
FED state: ↑↑ [Glucose], so there is no need to continue to make more Glucose by Glycogen breakdown
Glucose allosterically inhibits glycogen phosphorylase, from producing glucose -1-phosphate, with no effect on Glycogen Synthase
This is of massive importance for the liver
allosteric regulation of enzymes is _____the cell regulation
within
allosteric regulation involves binding of what to what
of an effector molecule to the enzyme and thereby altering its activity.
covalent regulation of metabolic processes is usually in response to what and give ex
extracellular stimulus,
such as from a hormone which then binds to an extracellular receptor, and which leads to a cascade of biochemical events leading to an effect on the target enzyme(s).
what are the 6 components of hormone action
- Signal
- Receptor
- Coupling
- Amplification
- Effect
- Termination
2,3 and 4 are all to do with signal transduction
name the 3 categories of signal hormones and give ex
Peptides or polypeptides: e.g. Glucagon and Insulin
Steroid hormones: e.g. Glucocorticoids, sex steroid hormones
Amino Acid derivatives: e.g. T3 and T4, Catecholamines (e.g. Adrenaline
explain the use of receptors
Hormones cannot stimulate a metabolic process directly.
They have, in the first instance, to bind to a specific receptor
- Steroids - Intracellular receptors interact with chromatin and effect mRNA transcription – not discussed further.
- Others – Bind to receptors located on the external surface of the plasma membrane
explain extracellular receptors
theyre usually glycoproteins
N-linked region contains oligosaccharides on the extracellular surface – these convey specificity
Receptor distribution – dependent upon the tissue
is Binding of a hormone to its receptor sufficient on its own to affect metabolic pathways.
no
Hormone-Receptor binding is coupled to what
an intracellular event
explain Coupling
Peptide and amino acid hormone receptors are coupled to a specific ‘Guanyl-stimulatory binding protein’ (GS-protein) on the intracellular surface of the plasma membrane
what re the 3 subunits GS consist of
GS consists of 3 subunits:
- α subunit (45 kDa)
- β subunit (35 kDa)
- γ subunit (7kDa)
explain alpha G protein subunits
α-subunit can interconvert between a form which binds GDP and a form that binds GTP depending on whether there is a hormone signal or not.
When there is no hormone signal the α-subunit ‘rests’ in the GDP binding form and there is no interaction between the unoccupied receptor and GS-protein.
When there is a hormone signal, the α-subunit changes its conformation and loses the GDP and instead binds GTP.
The α-GTP subunit dissociates from the rest of the GS-protein and binds to and activates an enzyme called adenylate cyclase which is located within the plasma membrane.
In the absence of further hormone stimulation, the GTP is hydrolysed to GDP and the α-GDP subunit dissociates from adenylate cyclase and re-associates with the rest of the GS-protein subunits.
explain the beta and gama subunits in G proteins
The βγ subunit does not undergo a conformational change, and acts as its own signalling molecule activating and inhibiting various enzymes
how is the formation of cyclic AMP (cAMP) from ATP catalysed
as When bound to the α-GTP subunit of the GS-protein, adenylate cyclase is activated and is able to catalyse the formation
how many molecules of cAMP are produced for every activated molecule of adenylate cyclase
hundreds
this is the amplification of the hormone signal.
cyclic AMP is also called what
a second messenger
cAMP is a potent activator what another enzyme
protein kinase A
cAMP-dependent protein kinase
how is Protein Kinase A a tetramer
2 regulatory subunits
2 catalytic subunits
The two free catalytic subunits on protein kinase A catalyse what
catalyse the phosphorylation of specific serine or threonine residues on target proteins
- The phosphorylated proteins may activate or inactivate enzymes(as is the case with the regulation of glycogen metabolism) ormodulate the activity of cellular ion channels.
- Protein kinase A can phosphorylate specific proteins that bind topromoter regions of DNA, causing increased expression ofspecific genes.
explain what happens during termination
Loss of hormone signal
Dephosphorylation of proteins
- The phosphate groups added to proteins by protein kinases areremoved by the actions of phosphoprotein phosphatases, enzymesthat hydrolytically cleave phosphate esters.
- This ensures that changes in enzymatic activity induced by proteinphosphorylation are not permanent.
Hydrolysis of cAMP
- cAMP is readily hydrolysed to 5’-AMP byphosphodiesterase.
- 5’-AMP is not an intracellular signalling molecule. [Note:Phosphodiesteraseis inhibited bymethylxanthinederivatives such ascaffeine.]
give a Physiological Example of Inhibition of synthesis by a cAMP cascade
Target enzyme – GlycogenSynthase
Glycogen synthase exists in 2 forms:
- “a” form, which isNOTphosphorylated and is the most activeform.
- “b” form, whichISphosphorylated and inactive.
Glycogen synthase a is converted to the b form (andtherefore inactivated) by phosphorylation’s ata numberofsites on theenzyme.
The level of inactivation is proportional to its degree ofphosphorylation.
Binding of the hormone glucagon or adrenaline tohepatocyte receptors, or adrenaline to muscle cell receptors,results in the activation ofadenylatecyclase.
cAMP is synthesised which activates protein kinase A.
- Protein kinase A phosphorylates glycogen synthase a toglycogen synthase b, and therefore inactivates glycogensynthesis.
Glycogen synthase b can be transformed back to glycogensynthase a by phosphoprotein phosphatase type 1, whichremoves the phosphate groups hydrolytically.
explain the physiological example of a Activation of breakdown by a cAMP cascade
The binding of glucagon or adrenaline to receptors signalsthe need for glycogen to be degraded – either to elevateblood glucose levels (contributed by liver glycogen) or toprovide energy in exercising muscle.
Activation of protein kinase A
Activation of phosphorylase kinase
Active protein kinase A phosphorylates the inactive form ofphosphorylase kinase, resulting in its activation.
Activation of glycogen phosphorylase
Glycogen phosphorylase exists in an inactive “b” form and an active“a” form.
Active phosphorylase kinase phosphorylates glycogen phosphorylaseb, converting it into active glycogen phosphorylase a, which beginsglycogen breakdown.
Phosphorylase a is reconverted to phosphorylase b byphosphoprotein phosphatase type 1.
give a Summary of the reciprocal regulation of glycogen synthesis and degradation
1- Glycogen synthesis and degradation are regulated by the same hormonal signals:
An elevated insulin level results in overall increased glycogen synthesis, whereas elevated glucagon (or adrenaline) levels cause increased glycogen degradation.
2- Cyclic AMP levels fluctuate in response to hormonal stimuli:
cAMP levels in cells increase in response to hormonal stimuli, e.g. glucagon and adrenaline in liver and adrenaline in muscle.
cAMP levels decrease in the presence of insulin.
3- Key enzymes are phosphorylated by a family of kinases, only some of which are cAMP dependent:
Phosphorylation of an enzyme causes a conformational change that affects the active site. This can greatly increase the catalytic activity of some enzymes or decrease it for others.
Muscles contract because of what
because of Ca2+ release from sarcoplasmic reticulum.
explain the Phosphorylase kinase can also be activated allosterically in muscle
Ca2+ binds to a subunit of phosphorylase kinase called Calmodulin.
Ca2+-Calmodulin activates phosphorylase kinase, thereby activating glycogen phosphorylase and hence causing glycogen breakdown. Glucose released fuels muscle contractions.
When muscle relaxes, Ca2+ returns to the sarcoplasmic reticulum and the phosphorylase kinase becomes inactive and glycogen phosphorylase a is converted to the inactive, glycogen phosphorylase b.
