Metabolic integration
Shuttles, exchange reactions and alternate fuels that connect otherwise separated compartments and pathway neighborhoods.
Metabolic integration is dominated by coupled systems. The malate–aspartate shuttle combines dehydrogenases, aminotransferases and antiporters so that cytosolic reducing equivalents influence the matrix NADH pool without NADH itself crossing the inner membrane.
Ketone bodies package acetyl-derived carbon into water-soluble molecules. Their production and use occur in different physiological contexts and enzyme complements; a shared bloodstream does not make every tissue a net producer.
Carbon dioxide hydration and inositol-phosphate transport illustrate two different integration problems: rapid acid–base interconversion in defined compartments, and movement of highly phosphorylated signals among cytosol, nucleus and endoplasmic reticulum.
Ideas to carry into the map
- Shuttles transfer equivalents through coupled reactions.
- Ketone production and utilization are not tissue-equivalent.
- Transport events are part of pathway chemistry.
Pathways in this field note
A coupled transport-and-reaction system that transfers cytosolic reducing equivalents into the mitochondrial matrix without moving NADH itself.
Hepatic formation and extrahepatic use of water-soluble acetyl-unit carriers.
Compartment-specific interconversion of carbon dioxide and hydrogencarbonate by carbonic anhydrases.
Phosphorylation, dephosphorylation and transport of inositol phosphates across cytosol, nucleus and endoplasmic reticulum.