Metabolic Regulation

What regulates glycolysis and gluconeogensis?

Fructose 2,6-bisphosphate is responsible for regulating both glycolysis and gluconeogenesis and serves as a switch to ensure that both pathways are not occuring at the same time. When F2,6-BP is high, PFK-1 is turned on/activated and FBPase is turned off/inhibited (↑ F2,6-BP = ↑ PFK-1 = ↓ FBPase)

How are metabolic pathways regulated by covalent modification?

Protein activity can be modulated by post-translational modifications such as phosphorylation by kinases or dephosphorylation by phosphatases, shape = function so if shape changes, it can affect whether an enzyme is turned on or not (other modifications can include acetylation, methylation, glycosylation, etc.)

How are metabolic pathways regulated by synthesis/degradation?

The amount of enzyme present in a cell necessarily affects its metabolic activity, changing the amount of enzyme present in a cell through protein synthesis or degradation (often induced by hormones) can affect whether metabolic activity is high or not, signal takes a long time to have an effect but is also long-lasting

How is glycolysis regulated through hexokinase?

G6-P inhibits hexokinase I-III (expressed in all tissues except the liver, product inhibition), liver hexokinase IV is NOT inhibited by G6-P allowing the liver to uptake extra glucose

What is the most regulated step in glycolysis?

Step 3, the committed step, done through phosphofructokinase-1 (PFK-1)

How is glycolysis regulated through PFK-1?

PFK-1 is inhibited by ATP (a substrate of that step), PFK-1 is activated by F2,6-BP, it is also activated by AMP (good indicator of cells overall energy state, ↑ AMP = ↓ ATP = PFK-1 activates to make more ATP, AMP and ATP are inversely related)

How is F2,6-BP formed?

F2,6-BP is formed from F6-P by PFK-2, reverse reaction is catalyzed by FBPase-2, both activities are present on the same enzyme and their activity is tightly regulated by phosphorylation to ensure that both activities are not occuring simultaneously

What is signal transduction?

When a extracellular ligand binds to a G protein-coupled receptor bound to the membrane, the induced fit results in a conformational change in the receptor which activates and releases an intracellular G protein which will then go and bind to an effector enzyme which will stimulate/inhibit its activity by modifying secondary messengers (transduced signals may be amplified)

Why is signal transduction favorable?

It is a good way for cells to receive messages without the signals having to physically enter the cell

What is the primary messenger in signal transduction?

The external stimulus that will bind to the membrane receptor

How does the membrane receptor communicate to the effector enzyme?

A transducer can move along the inner surface of the membrane to reach the effector enzyme (typically bound to the membrane by a long hydrocarbon tail)

What are G-protein receptors?

They are protein complexes of a receptor and the multi subunit G-protein, α, β, and γ

What are the steps for the G-protein cycle (regulation)?

1. Resting state: G protein exists as αβγ trimer with GDP bound to α

2. Activation: Ligand binds G-protein coupled receptor (GPCR) → receptor activates G protein

3. GDP → GTP exchange: α releases GDP and binds GTP

4. Dissociation: α–GTP separates from βγ (both can signal)

5. Effector interaction: α–GTP and/or βγ regulate target proteins

6. Termination: α hydrolyzes GTP → GDP

7. Reassociation: α–GDP recombines with βγ → inactive trimer restored

What returns the α-GTP complex to its inactive form?

Native GTPase activity hydrolyzes the α-GTP complex back to α-GDP returning it to its inactive form

What must occur for the G-protein (αβγ trimer) to activate?

GDP must leave and GTP must bind the α subunit and the α subunit + βγ complex must dissociate from the receptor

What modulates the activity of adenylate cyclase?

The active G-protein (α or βγ complex)

What does adenylate cyclase do?

It catalyzes the conversion of ATP to cyclic AMP (cAMP)

What is the purpose of cAMP?

It is an important secondary messenger which subsequently regulates the activity of several key regulatory enzymes including protein kinase A (PKA)

What does the structure of protein kinase A (PKA) look like?

PKA is a heterotetramer of two regulatory and two catalytic domains, when the catalytic domains are attached to the regulatory domains, they are inactive

What activates protein kinase A (PKA) and how?

cAMP, it binds to the regulatory domain resulting a conformational change which causes the regulatory domains to lose affinity for the catalytic domains, resulting in their release, which in turn activates them

What does activated protein kinase A (PKA) do?

It is responsible for phosphorylating a variety of regulatory proteins

What inactivates cAMP?

Phosphodiesterase hydrolyzes cAMP to AMP, inactivating the signal response

What is the general path for epinephrine signaling?

Ephinephrine or norepinephrine (primary messenger) bind to β-Adrenergic receptor (membrane receptor) which activates the α-GTP complex, which moves along the inner surface of the membrane until it reaches adenylate cyclase (effector enzyme) which catalyzes the conversion of ATP to cAMP, cAMP will then activate PKA which will activate glycogen phosphorylase by phosphorylating it

What is the purpose of glycogen phosphorylase (epinephrine signaling pathway)?

It breaks down glycogen to increase blood glucose levels which will allow more energy to be available for a fight or flight response

How does caffeine affect cAMP?

It inhibits phosphodiesterase which will result in cAMP levels persisting which will result in epinephrine (increased ATP levels) persisting, it will also block adenosine receptors in the brain inhibiting the sedative effects of adenosine on the CNS

How does PFK-2 get turned on/off?

When glucagon binds to its receptor (↑ glucagon = ↓ glucose), adenylate cyclase activates cAMP which activates PKA, PKA phosphorylates PFK-2, inactivating it, which simultaneously activates FBPase-2, as a result, F2,6-BP levels drop and F6-P levels rise stimulating gluconeogenesis

Why is PFK-2/FBPase-2 only relevant to liver cells?

Because gluconeogensis can only occur in liver cells, the regulation of the whether gluconeogensis or glycolysis is turned on only matters in the liver

What process is occuring is PFK-2 is turned on and FBPase-2 is turned off?

Glycolysis

How is pyruvate kinase regulated?

It is regulated allosertically by feed-forward activation from F1,6-BP and by product inhibition from ATP, it is also inactivated with phosphoylated by PKA