BIOCHEM 5614-- 15 Part 1: Regulations Of Metabolic Pathways

MAIN POINT: Explains how cells turn metabolic pathways on/off using inhibitors, activators, hormones, and signals crucial for energy management and adaptation

HEXOKINASE

  • Hexokinase is regulated by glucose 6 phosphate. When theres an influx of glucose 6 phosphate, it inhibits hexokinase to stop producing more of it (feedback inhibition). Influx of glucose 6 phosphate signals that glycolysis is occuring to convert glucose into pyruvate for energy, ATP. When there is high amounts of G6P already, there isn’t a need to make more energy from glucose so hexokinase would be inhibited. This is to prevent unecessary ATP use when energy or intermediates are already abundant (G6P accumulation also means that downstream enzymes like PFK 1 are slowed due to High ATP)

  • However, G6P only inhibits hexokinase 1-3, not 4 (liver)

  • The reason hexokinase 4 isn’t regulated is because it has a lower affinity for glucose compared to the other isoforms. Lower affinity means that it’ll take higher amounts of glucose for hexokinase 4 to be more active (like after a meal). This means that theres no point in regulating it with G6P because it only has high activity when glucose level is high.

  • This property allows liver to store excess glucose as glycogen when other cells already have enough glucose.

  • Hexokinase regulation ensures that tissues don’t waste energy phosphorylating glucose when it’s not needed.

  • The liver’s unique glucokinase system allows it to act as a buffer for blood glucose, protecting the rest of the body from glucose spikes.

FRUCTOSE 6 PHOSPHATE:

  • Fructose-6-phosphate (F6P) does not directly inhibit glucokinase, but it promotes its inactivation through a regulatory protein called GKRP (glucokinase regulatory protein).

  • When F6P levels are high, GKRP binds to glucokinase (hexokinase 4) and sequesters it in the nucleus, away from the cytoplasm, where glucose phosphorylation occurs.

  • This prevents glucokinase from converting glucose → G6P, effectively downregulating glycolysis and glycogen synthesis.

  • High F6P levels often occur during low blood glucose or active gluconeogenesis (when the liver is producing glucose rather than using it).

  • Sequestering glucokinase prevents a futile cycle, ensuring the liver doesn’t simultaneously make and consume glucose.

  • When glucose levels rise — typically after a meal — these molecules disrupt the GKRP–glucokinase complex.

  • Glucokinase is then released back into the cytoplasm, where it can phosphorylate glucose for glycolysis or glycogen storage.

PHOSPHOFRUCTOKINASE 1

  • PFK 1 is highly regulated because it is at the committed step of glycolysis.

  • This is the committed and rate-limiting step of glycolysis — once this reaction happens, the pathway is committed to energy production (can’t easily reverse to glucose).

What activates it:

ADP, AMP, Fructose 2,6-bisphosphate

What inhibits it:

Citrate, ATP, Phosphoenolpyruvate

Why would ATP inhibit it (substrate) and ADP/ AMP (product) activate it? Isn’t it counterintuitive?

  • It seems counterintuitive because ATP is a substrate — but it’s also the final energy product of glycolysis.
    The cell uses ATP at low concentrations for catalysis (substrate role), but at high concentrations, ATP binds to an allosteric site distinct from the catalytic site to turn the pathway down.
    ADP and AMP compete for this site and relieve inhibition, ensuring glycolysis runs only when energy is low.

FRUCTOSE 2,6-BISPHOSPHATE

  • F2,6BP is made and degraded by a bifunctional enzyme

  • When glycolysis occurs, F6P is converted to F26BP by phosphofructokinase 2. F26BP would then activate PFK1 to commit into glycolysis step by converting F6P into F16BP

  • Reverse can occur where F26BP is converted to F6P by Fructose 2,6 Bisphosphatase

  • When theres high amounts of F26BP, PFK1 is activated (commit to glycolysis)

  • When theres high amounts of F26BP, F16BPase is inactivated (prevent gluconeogensis)

  • F2,6BP is a powerful allosteric regulator, not an intermediate of glycolysis or gluconeogenesis.

  • It activates PFK-1 (stimulates glycolysis) and inhibits Fructose-1,6-bisphosphatase (F1,6BPase) (blocks gluconeogenesis).

  • This ensures the liver does not run both pathways at the same time (prevents a futile cycle).

G PROTEIN RECEPTORS

A receptor complex made up of a receptor domain, and proteins alpha, beta, gamma

  • A GPCR is a membrane receptor with seven transmembrane α-helices.

  • It is coupled to a heterotrimeric G-protein complex, composed of α, β, and γ subunits (not “upsilon”).

  • In its inactive state, the α-subunit is bound to GDP and associated with the βγ complex.

2. Activation by First Messenger

  • When a first messenger (e.g., epinephrine, glucagon) binds to the receptor’s extracellular domain, it triggers a conformational change in the receptor.

  • This change promotes exchange of GDP for GTP on the α-subunit, activating it.

3. Signal Transduction

  • The α-GTP subunit then dissociates from the βγ complex and interacts with a target effector enzyme—most commonly adenylate cyclase.

  • The type of Gα determines the response:

    • Gs (stimulatory): activates adenylate cyclase → ↑ cAMP

    • Gi (inhibitory): inhibits adenylate cyclase → ↓ cAMP

4. Second Messenger Cascade

  • Adenylate cyclase converts ATP → cyclic AMP (cAMP).

  • cAMP acts as a second messenger, binding to the regulatory subunits of Protein Kinase A (PKA).

  • This releases the catalytic subunits of PKA, which then phosphorylate target enzymes and transcription factors, amplifying the original hormone signal.

5. Signal Termination

  • The signal is self-limiting:

    • The α-subunit hydrolyzes GTP → GDP (its own GTPase activity), reassociating with βγ.

    • Phosphodiesterases degrade cAMP → AMP, turning off PKA activity.