Regulation of Metabolism

Metabolic regulation coordinates the activity of hundreds of enzymes to meet the cell's needs for energy, biosynthetic precursors, and homeostasis. Regulation operates at multiple levels: allosteric (milliseconds), covalent modification (seconds to minutes), and hormonal/transcriptional (minutes to hours).

Allosteric Regulation

Allosteric enzymes possess distinct regulatory sites separate from the active site. Binding of an effector molecule induces conformational change that alters catalytic activity. Most allosteric enzymes display sigmoidal (cooperative) substrate-velocity curves rather than hyperbolic Michaelis-Menten kinetics, enabling sharp, switch-like responses to small changes in metabolite concentration. This behaviour is described by the Hill equation: v = Vmax[S]^n / (K0.5^n + [S]^n), where n (Hill coefficient) > 1 indicates positive cooperativity.

Key allosteric regulatory principles: (1) End-product (feedback) inhibition — the final product of a pathway inhibits an early enzyme (e.g., ATP inhibits PFK-1; isoleucine inhibits threonine deaminase). (2) Feedforward activation — an early metabolite activates a downstream enzyme (e.g., fructose-1,6-bisphosphate activates pyruvate kinase). (3) Energy charge regulation — ATP/ADP/AMP ratio signals energy status to multiple enzymes. The adenylate kinase reaction (2ADP ⇌ ATP + AMP) means a small fall in ATP produces a proportionally larger rise in AMP, providing metabolic amplification.

Energy charge = (ATP + 0.5 ADP) / (ATP + ADP + AMP). High energy charge (>0.9) inhibits catabolic pathways and activates anabolic ones. Low energy charge (<0.7) does the reverse.

Covalent Modification: Phosphorylation/Dephosphorylation

Reversible phosphorylation is the dominant regulatory covalent modification. Protein kinases transfer the gamma-phosphate of ATP to serine, threonine, or tyrosine residues; protein phosphatases remove it. The outcome (activation vs inhibition) depends on the specific protein.

Examples: Glycogen phosphorylase: phosphorylation at Ser14 activates (phosphorylase a); dephosphorylation inactivates (phosphorylase b). Glycogen synthase: phosphorylation at multiple sites inhibits (synthase b); dephosphorylation activates (synthase a). Pyruvate dehydrogenase: phosphorylation inactivates; dephosphorylation activates. Acetyl-CoA carboxylase (ACC): AMPK phosphorylates Ser79, inactivating ACC and reducing malonyl-CoA, relieving CPT-I inhibition and enabling fatty acid oxidation.

Phosphorylation cascades amplify signals: 1 glucagon molecule → 1 adenylyl cyclase activated → many cAMP → many PKA molecules activated → many phosphorylase kinase molecules activated → many phosphorylase molecules activated → massive glycogenolysis. This amplification (>10^6-fold) allows nanomolar hormone concentrations to produce large metabolic effects.

Hormonal Control: Insulin Signalling

Insulin (released from pancreatic beta cells in response to elevated blood glucose) is the primary anabolic hormone. Receptor: insulin receptor tyrosine kinase (IRTK) — a heterotetrameric receptor with intrinsic tyrosine kinase activity. Binding of insulin causes receptor autophosphorylation on tyrosine residues, activating the kinase.

PI3K-Akt pathway: phosphorylated IRTK recruits IRS-1/IRS-2 (insulin receptor substrates), which recruit PI3-kinase (PI3K). PI3K phosphorylates PIP2 → PIP3 (PTEN opposes this). PIP3 recruits PDK1, which phosphorylates and activates Akt (PKB). Akt mediates most of insulin's metabolic effects: (a) Phosphorylates and inactivates GSK-3 → dephosphorylation of glycogen synthase → glycogen synthesis activated. (b) Promotes GLUT4 vesicle translocation to plasma membrane (in muscle and adipose) → glucose uptake. (c) Activates PDE3B → lowers cAMP → reduces PKA activity → reduces lipolysis and glycogenolysis. (d) Promotes protein synthesis via mTORC1. (e) Suppresses FOXO transcription factors → reduces gluconeogenic gene expression (PEPCK, G6Pase).

Hormonal Control: Glucagon Signalling

Glucagon (from pancreatic alpha cells in response to low blood glucose) opposes insulin. Receptor: glucagon receptor (GPCR) coupled to Gs → adenylyl cyclase activation → cAMP elevation → PKA activation. PKA effects: (a) Activates phosphorylase kinase → glycogenolysis. (b) Phosphorylates PFK-2/FBPase-2 at Ser32 → activates phosphatase domain → lowers F2,6-BP → reduces PFK-1 activity → reduces glycolysis. (c) Phosphorylates and inhibits pyruvate kinase → reduces glycolysis. (d) Activates hormone-sensitive lipase (HSL) in adipocytes → lipolysis → fatty acids available for beta-oxidation and ketogenesis. (e) Upregulates PEPCK and G6Pase transcription → promotes gluconeogenesis.

Epinephrine acts via beta-adrenergic receptors (cAMP/PKA) in liver and adipose, and via alpha1-adrenergic receptors (IP3/DAG/Ca2+) in muscle — both promote glycogenolysis and mobilise fuel.

AMP-Activated Protein Kinase (AMPK)

AMPK is the master energy sensor of the cell. It is activated allosterically by AMP and ADP, and by LKB1-mediated phosphorylation of Thr172 (AMP promotes LKB1 binding). AMPK is activated when the AMP:ATP ratio rises (exercise, fasting, ischaemia, metformin). AMPK switches cells from anabolic to catabolic mode:

Catabolic activation: phosphorylates and activates PGC-1α (mitochondrial biogenesis), activates fatty acid oxidation (phosphorylates/inactivates ACC, reducing malonyl-CoA, relieving CPT-I inhibition), activates glycolysis (phosphorylates PFKFB3, increasing F2,6-BP in heart/muscle), promotes GLUT4 translocation.

Anabolic inhibition: inactivates ACC (→ reduces fatty acid synthesis), inactivates HMG-CoA reductase (→ reduces cholesterol synthesis), inhibits mTORC1 (→ reduces protein synthesis), inhibits SREBP-1c (→ reduces lipogenic gene expression).

AMPK is a target of metformin (indirectly, via Complex I inhibition raising AMP:ATP) and may partly explain the metabolic benefits of exercise.

Fed vs Fasted State

Fed state (high insulin, low glucagon): glucose taken up by muscle (GLUT4) and liver (GLUT2). Liver: glycogen synthesis (glycogen synthase active), glycolysis active (high F2,6-BP), fatty acid synthesis (ACC active, malonyl-CoA high, CPT-I inhibited), triglyceride and VLDL synthesis. Muscle: glycogen synthesis, protein synthesis (mTORC1 active).

Fasted state (low insulin, high glucagon): liver maintains blood glucose via glycogenolysis (first ~12h) then gluconeogenesis (12h+). Adipose: lipolysis (HSL active) → NEFA and glycerol released. Liver: beta-oxidation (CPT-I active), ketogenesis. Brain: initially glucose-dependent; after several days, adapts to use beta-hydroxybutyrate as primary fuel (~60-70% of brain fuel). Muscle: switches from glucose to fatty acid oxidation; provides alanine and glutamine for hepatic gluconeogenesis.

Clinical Relevance

Type 2 diabetes: insulin resistance in muscle and adipose (defective IRS-1 signalling, reduced GLUT4 translocation), combined with inappropriate hepatic gluconeogenesis (FOXO not suppressed), leads to hyperglycaemia. Metformin activates AMPK and suppresses hepatic gluconeogenesis. SGLT2 inhibitors, GLP-1 analogues, and thiazolidinediones (PPAR-γ agonists) target different nodes of this regulatory network. Glycogen storage diseases reflect genetic disruptions of glycogen regulatory enzymes (Von Gierke — G6Pase deficiency; Hers — liver phosphorylase deficiency).