ABSTRACT
Metabolism involves coordinated biochemical reactions at different levels. These levels, in order of complexity, are as follows:
• Molecule + enzyme • Linear, branched, or cycling metabolic pathways and polyenzyme complexes • Cell compartment or organelle of microbial cell • Metabolic block of microbial cell • Metabolism of whole microbial cell • Metabolism of microbial population • Metabolism of microbial ecosystem • Microbial metabolism of whole biosphere
The fundamental unit of metabolism is the biochemical reaction. All biochemical reactions in cells are catalyzed by protein molecules called enzymes:
S + E(+C) ↔ [ES] ↔ [EP] ↔ P + E(+C)
where E is the enzyme, S is the substrate, ES is the unstable enzyme-substrate complex, EP is the unstable enzyme-product complex, and P is the product.
