regulation of metabolism Flashcards
To coordinate metabolic processes its vital some enzymes or subject to strict controls. Name factors involved in regulation of biochemical pathways.
Concentrations of substrates & products. Modifications. Endocrine signals. Other enzymes.
What did michealis & menten propose?
That the enzyme reversibly combines with its substrate to form an enzyme substrate complex that breaks down to product & regenerating free enzyme.
The michealis-menten equation describes how reaction velocity varies with substrate concentration, what is this equation?
Vo=Vmax(s)/Km+(s)
Vo = initial reaction velocity
Vmax= maximal velocity
Km = michealis constant
(s)= substrate concentration.
Explain a similarity in Km and (S).
Km & (S) are numerically equal at which the reaction velocity is at 1/2Vmax.
Km doesn’t vary with the concentration of enzyme. Small Km (numerically) reflects a high affinity of the enzyme for substrate as low concentration of substrate is needed to half-saturate the enzyme aka to reach 1/2Vmax. These enzymes are targeted for regulation. Explain High Km.
A numerically high km reflects a low affinity of the enzyme for substrate as a high concentration is required to half saturate enzyme eg. to reach 1/2Vmax.
when (S) is much greater than Km, velocity is constant & equal to Vmax. Rate of reaction is independent of (s) and is said to be ‘zero order’ with respect to substrate concentration. Explain if (s) is much less than Km.
Velocity of reaction is proportional to (s).The rate is said to be ‘first order’ with respect to substrate.
what do effectors do?
They are molecules that bind non-covalently to an enzyme & inhibit its activity.
What are the 2 types of effectors & explain them.
‘Homotropic’, usually positive, when the substrate is an effector. ‘Heterotrophic’, when effector is not the substrate, may have stimulartory or inhibitory effect.
Processes of glycogen synthesis & glycogen breakdown are both regulated in what ways?
Allosterically, covalently and by induction/repression of synthesis.
Name the enzyme for glycogen synthesis.
Glycogen synthase.
Name the enzyme for glycogen breakdown.
Glycogen phosphorylase.
What process is the fed state, and which the fasted?
Fed state = glycogen synthesis (glucose to glycogen)
fasted state = Glycogen breakdown (Glycogen to glucose)
What molecule activates glycogen synthase and inhibits glycogen phosphorylase?
glucose 6-phosphate.
Adenosine monophosphate (AMP) has no effect on ___ but allosterically activates ___. Important in exercising ___ muscle
glycogen synthase, glycogen phosphorylase. Skeletal.
ATP allosterically inhibits ___, but has no effect on ___
glycogen phosphorylase, glycogen synthase.
AMP & ATP both don’t have an effect on what enzyme?
glycogen synthase.
If fed, more glucose does not need to be made by glycogen breakdown….
so glucose allosterically inhibits glycogen phosphorylase, with no effect on glycogen synthase. This is important in liver.
What does Allosteric mean?
Happens In the cell. Involves binding of an effector molecule to the enzyme, altering its activity.
There are 6 components of hormone action to consider before looking at glycogen synthase and phosphorylase.
Signal, receptor, coupling, amplification, effect, termination.
In terms of the signal; what are the 3 categories of hormones?
1) Peptide or polypeptides e.g glucagon & insulin.
2) steroid hormones e.g. glucocorticoids, sex steroid hormones.
3) Amino acid derivatives. e.g T3, T4 Catecholamines (e.g-adrenaline)
Hormones must bind to receptors in first step for metabolic processes. Where do the hormones bind to receptors?
Steriods; intracellular receptors interact with chromatin and effect mRNA transcription.
Others; bind to receptors on surface of plasma membrane.
Extracellular receptors are usually glycoproteins.
N-linked region contains oligosaccharides on the extracellular surface- these convey speciality.
peptide and amino acid hormone receptors are coupled to a specific what?
Guanyl-stimulatory binding protein (Gs-protein), on the intracellular surface of the plasma protein.
Guanyl protein consists of 3 subunits, what are they?
a subunit (45kDa),
Beta subunit (35kDa)
y subunit (7kDa)
When there is no hormone signal, where does the a-subunit rest?
In the GDP & there are no interactions between receptor & Gs-protein.
If there is a hormone signal, what happens to subunit a?
It changes its conformation, loses the GDP and binds GTP. This a-GTP subunit dissociates from the Gs-protein & binds/activates enzyme ‘adenylate cyclase, located in plasma membrane.
What happens if there is no further hormones stimulation?
GTP is hydrolysed to GDP, the a-GDP subunit dissociates from adenylate cyclase and re-associates with rest of the Gs-protein subunits.
when bound to the a-GTP subunit of the Gs-protein, adenylate cyclase is activated & able to catalyse the formation of what?
Cyclic AMP (cAMP) from ATP. 100s molecules of cAMP are produced for every activated molecules of adenylate cyclase.
cyclic AMP is known as what?
a second messenger.
what enzyme is cAMP a potent activator of?
Protein kinase A.
Describe and explain protein kinase a.
A tetramer, 2 regulatory subunits & 2 catalytic subunits. The catalytic subunits catalyse the phosphorylation of specific serine or threonine residues on target proteins.
The phosphorylated proteins may activate or inactivate enzymes.
Protein kinase A can phosphorylate specific proteins that bind to promoter regions of DNA causing what?
Causing increased expression of specific genes.
Termination; loss of hormone signal. Explain dephosphorylation of proteins.
The phosphate groups added to proteins by protein kinases are removed by the actions of ‘phosphoprotein phosphates’, enzymes that hydrolytically cleave phosphate esters. Changes in enzymatic activity induced by protein phosphorylation are not permanent.
Explain hydrolysis of cAMP.
cAMP is readily hydrolysed to 5’-AMP by phosphodiesterase. 5’-AMP is not an intracellular signalling molecule. Note that phosphodiesterase is inhibited by methylxanthine derivatives such as caffeine.
Explain inhibition of glycogen synthesis by a cAMP cascade.
Glycogen synthase a (NOT phosphorylated & most active form) is converted to b form (Is phosphorylated and inactive) by phosphorylations at a number of sites on the enzyme.
Binding of glucagon or adrenaline to hepatocyte/ muscle cell receptors results in activation of what?
Adenylate cyclase.
cAMP is synthesised which activates protein kinase A. Protein kinase a phosphorylates glycogen synthase a to b therefore inactivates glycogen synthesis.
Glycogen synthase B can be transformed back to glycogen synthase a by phosphoprotein phosphatase type 1, which removes the phosphate group hydrolytically.
the binding of glucagon or adrenaline to receptors signals the need for what?
For glycogen to be degraded- either to elevate blood glucose levels or to provide energy in exercising muscles.
protein kinase a phosphorylates the inactive form of what resulting in its activation.
In phosphorylase kinase.
Active phosphorylase kinase phosphorylates glycogen phosphorylase b to a which begins what?
Glycogen breakdown.
cAMP levels decrease when?
when in the presence of insulin.
