regulation of metabolism

Question 1

why do we need strict control of some enzymes involved in metabolic processes

Answer 1

because To co-ordinate metabolic processes, it is vital

Question 2

name some factors that may be involved in the regulation of biochemical pathways

Answer 2

Concentrations of substrates and products

Modifications

Endocrine signals

Other enzymes

Question 3

General Mechanisms of Enzyme Regulation is done by what

Answer 3

the Michaelis-Menten Equation

Question 4

Leonor Michaelis and Maud Menten proposed what

Answer 4

proposed that the enzyme reversibly combines with its substrate to form an ES complex that subsequently breaks down to product, regenerating free enzyme.

Question 5

The Michaelis-Menten equation describes what

Answer 5

how reaction velocity varies with substrate concentration

Question 6

explain some assumptions of Michaelis-Menten Equation

Answer 6

[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.

Question 7

The Michaelis constant (Km) is characteristic for what and what does this reflect

Answer 7

for an enzyme and its substrate and reflects the affinity of the enzyme for that substrate.

Question 8

Km is numerically equal to what

Answer 8

equal to [S] at which the reaction velocity is at ½Vmax. ​

Question 9

Km does not vary with what of an enzyme

Answer 9

conc of an enzyme

Question 10

explain small Km

Answer 10

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.​

Question 11

explain high Km

Answer 11

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.

Question 12

what is rate of reaction proportional to

Answer 12

to the enzyme concentration at all [S].

Question 13

what does it mean when [s] is much greater than Km in a reaction

Answer 13

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.

Question 14

what does it mean When [S] is much less than Km in a reaction

Answer 14

the velocity of the reaction is roughly proportional to [S]. The rate is said to be first order with respect to substrate.

Question 15

what are effectors

Answer 15

molecules that may bind non-covalently to an enzyme and inhibit its activity.​

Question 16

name and explain the 2 types of effector

Answer 16

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

Question 17

explain Induction and Repression of Synthesis

Answer 17

Represents adaptive regulation whereby enzyme synthesis is either enhanced or decreased by certain physiological situations.​

Slow (days) mechanism of regulation.

Question 18

what is an example of induction and repression synthesis

Answer 18

glycogen metabolism

as Processes of glycogen synthesis and glycogen breakdown are reciprocally regulated allosterically, covalently, and by induction/repression of synthesis

Question 19

what are the 2 key regulatory enzymes in glycogen metabolism

Answer 19

SYNTHESIS: Glycogen Synthase​

BREAKDOWN: Glycogen Phosphorylase

Question 20

in a fed state glucose in broken down to what

Answer 20

glycogen

activated by glycogen synthesis

inhibited by glycogen breakdown

Question 21

in a fasted state glycogen in broken down into what

Answer 21

glucose

activated by glycogen breakdown

inhibited by glycogen synthesis

Question 22

what causes allosteric regulation of glycogen metabolism and explain how

Answer 22

glucose -6 - phosphate

as Glucose 6-phosphate activates Glycogen Synthase and inhibits Glycogen Phosphorylase

Question 23

what affect does AMP have on glycogen synthase

Answer 23

has no effect on Glycogen Synthase, but allosterically activates Glycogen phosphorylase – very important in exercising skeletal muscle

Question 24

in glycogen metabolism when is AMP used and what does it generate

Answer 24

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)

Question 25

explain ATP in terms of allosteric regulation of glycogen metabolism

Answer 25

↑↑ [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)

Question 26

explain glucoses effect in allosteric regulation of glycogen metabolism

Answer 26

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

Question 27

allosteric regulation of enzymes is _____the cell regulation

Answer 27

within

Question 28

allosteric regulation involves binding of what to what

Answer 28

of an effector molecule to the enzyme and thereby altering its activity.

Question 29

covalent regulation of metabolic processes is usually in response to what and give ex

Answer 29

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).​

Question 30

what are the 6 components of hormone action

Answer 30

1. Signal
2. Receptor
3. Coupling
4. Amplification
5. Effect
6. Termination

2,3 and 4 are all to do with signal transduction

Question 31

name the 3 categories of signal hormones and give ex

Answer 31

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

Question 32

explain the use of receptors

Answer 32

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​

Question 33

explain extracellular receptors

Answer 33

theyre usually glycoproteins​
N-linked region contains oligosaccharides on the extracellular surface – these convey specificity​
Receptor distribution – dependent upon the tissue

Question 34

is Binding of a hormone to its receptor sufficient on its own to affect metabolic pathways.​

Answer 34

no

Question 35

Hormone-Receptor binding is coupled to what

Answer 35

an intracellular event

Question 36

explain Coupling

Answer 36

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​

Question 37

what re the 3 subunits GS consist of

Answer 37

GS consists of 3 subunits:​
- α subunit (45 kDa)​
- β subunit (35 kDa)
- γ subunit (7kDa)

Question 38

explain alpha G protein subunits

Answer 38

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.

Question 39

explain the beta and gama subunits in G proteins

Answer 39

The βγ subunit does not undergo a conformational change, and acts as its own signalling molecule activating and inhibiting various enzymes

Question 40

how is the formation of cyclic AMP (cAMP) from ATP catalysed

Answer 40

as When bound to the α-GTP subunit of the GS-protein, adenylate cyclase is activated and is able to catalyse the formation

Question 41

how many molecules of cAMP are produced for every activated molecule of adenylate cyclase

Answer 41

hundreds

this is the amplification of the hormone signal.

Question 42

cyclic AMP is also called what

Answer 42

a second messenger

Question 43

cAMP is a potent activator what another enzyme

Answer 43

protein kinase A

cAMP-dependent protein kinase

Question 44

how is Protein Kinase A a tetramer

Answer 44

2 regulatory subunits
2 catalytic subunits

Question 45

The two free catalytic subunits on protein kinase A catalyse what

Answer 45

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.

Question 46

explain what happens during termination

Answer 46

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.]

Question 47

give a Physiological Example of Inhibition of synthesis by a cAMP cascade

Answer 47

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.

Question 48

explain the physiological example of a Activation of breakdown by a cAMP cascade

Answer 48

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.

Question 49

give a Summary of the reciprocal regulation of glycogen synthesis and degradation

Answer 49

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.

Question 50

Muscles contract because of what

Answer 50

because of Ca2+ release from sarcoplasmic reticulum.

Question 51

explain the Phosphorylase kinase can also be activated allosterically in muscle

Answer 51

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.