Biochemistry: Glycolysis, CAC, and Electron Transport Chain

Insulin

Stimulates GLUT4-containing vesicles to fuse with the cell membrane in muscle and adipose tissue, increasing glucose uptake.

GLUT4

An insulin-dependent glucose transporter found in skeletal muscle and adipose tissue.

Key regulated enzymes of glycolysis

Hexokinase, phosphofructokinase-1 (PFK-1), and pyruvate kinase.

Hexokinase regulation

Allosteric inhibition by glucose-6-phosphate, genetic control adjusting expression with metabolic needs, and substrate cycle balanced with glucose-6-phosphatase (in liver).

Glucose-6-phosphatase

Enzyme that converts G6P to free glucose, allowing glucose to leave the liver during glycogenolysis.

Hexokinase vs. Glucokinase

Hexokinase: high affinity (low Km), active in muscle; Glucokinase: low affinity (high Km), active in liver.

PFK-1 regulation

Inhibitors: ATP, citrate, low pH; Activators: AMP, ADP, fructose-2,6-bisphosphate (F2,6BP).

Fructose-1,6-bisphosphatase (FBPase) regulation

Inhibitors: AMP & F2,6BP; Activators: ATP.

PFK-1 committed step

It commits glucose to glycolysis and is regulated by the cell's energy state.

F2,6BP regulation

Insulin activates PFK-2, increasing F2,6BP to stimulate PFK-1; Glucagon activates FBP-2, decreasing F2,6BP to inhibit PFK-1.

Glucose-6-phosphate (G6P)

The branching point connecting glycolysis and glycogen synthesis.

UDP-glucose formation

The committed step of glycogen synthesis, where activation of G1P with UTP forms UDP-glucose.

Activation of glucose with UTP

Provides energy and a good leaving group for glycogen synthase to form glycosidic bonds.

Glycogen synthase

Creates α-1,4-glycosidic bonds.

Glycogen phosphorylase

Breaks α-1,4 bonds to release glucose-1-phosphate during glycogenolysis.

Dephosphorylation of G6P

Only performed in the liver due to the presence of glucose-6-phosphatase, allowing free glucose release to blood.

Pyruvate dehydrogenase complex (PDC)

Enzyme that converts pyruvate to acetyl-CoA.

PDC reaction reactants

Pyruvate, NAD⁺, CoA.

PDC reaction products

Acetyl-CoA, NADH, CO₂.

Thioester bonds

Store large negative free energy, driving the CAC forward.

Citric Acid Cycle (CAC)

Produces NADH and supplies acetyl-CoA for the CAC.

Turns of the CAC per glucose

Two turns (one per pyruvate).

Products of one turn of the CAC

3 NADH, 1 FADH₂, 1 GTP, 2 CO₂, regenerates oxaloacetate.

Amphibolic

CAC participates in both catabolism (energy production) and anabolism (biosynthesis).

Relationship between oxaloacetate and glycolysis/amino acids

OAA can become PEP → glucose; OAA can be produced from pyruvate; OAA can be formed from aspartate.

Citrate and lipid synthesis

Citrate exported to cytosol is converted to acetyl-CoA for fatty acid synthesis.

CAC intermediates replenished by anaplerotic reactions

OAA, α-ketoglutarate, succinyl-CoA, fumarate, malate.

Location of CAC enzymes

Mitochondrial matrix (except succinate dehydrogenase in inner membrane).

Committed step of CAC

Step 1: Citrate synthase (acetyl-CoA + OAA → citrate).

GTP generation in CAC

Step 5: Succinyl-CoA synthetase.

CoA consumption/regeneration in CAC

Consumed: Step 1 (citrate formation); Regenerated: Step 5 (succinyl-CoA → succinate).

CO₂ release in CAC

Steps 3 and 4.

Electron carriers reduction in CAC

NADH: Steps 3, 4, 8; FADH₂: Step 6.

Steps regulated by substrate availability in CAC

Step 1 depends on oxaloacetate availability.

Steps regulated by feedback inhibition in CAC

NADH inhibits steps 3 and 4.

Steps regulated by allosteric activation in CAC

ADP activates isocitrate dehydrogenase (step 3).

Steps regulated by competitive inhibition in CAC

Succinyl-CoA inhibits α-ketoglutarate dehydrogenase.

Coordination between glycolysis and CAC

By ATP/ADP and NADH levels that regulate PFK-1 and key CAC enzymes.

Main electron carriers in ETC

NAD⁺/NADH, FAD/FADH₂, ubiquinone (Q), cytochrome C.

Path of electrons from NADH in ETC

Complex I → Q → Complex III → cytochrome C → Complex IV → O₂.

Path of electrons from FADH₂ in ETC

Complex II → Q → Complex III → cytochrome C → Complex IV → O₂.

FADH₂ ATP production

It bypasses Complex I, pumping fewer protons.

CAC step overlapping with ETC

Step 6 (succinate dehydrogenase) = ETC Complex II.

Electron movement and ATP synthesis

Electrons move from high → low energy, pumping H⁺ that generate the proton motive force.

ETC complexes that pump protons

Complex I, III, and IV.

ATP synthase function

Protons flow down their gradient through the Fo unit, causing rotation that drives ATP formation in the F1 unit.

Oxidative phosphorylation vs substrate-level phosphorylation

Oxidative uses proton gradients; substrate-level transfers phosphate directly from a high-energy substrate.