Catalytic Efficiency of Conserved Metabolic Enzymes Across Evolutionary Complexity

Metabolic enzymes are highly conserved across life, yet their catalytic efficiencies vary among organisms of differing complexity. We compiled turnover numbers (kcat) and catalytic efficiencies (kcat/Km) for key enzymes in glycolysis, the TCA cycle, and oxidative phosphorylation from bacteria to mammals, drawing on BRENDA, BioNumbers, and primary literature. Rather than a universal trend, we find enzyme-specific and pathway-dependent patterns: TCA cycle dehydrogenases (citrate synthase, malate dehydrogenase, isocitrate dehydrogenase) consistently show 3-4x higher kcat in E. coli than in mammals, whereas glycolytic kinases and transferases show the opposite pattern--human RBC pyruvate kinase ([~]1,375 s-1) is 2.8x faster than E. coli ([~]497 s-1), and yeast GAPDH ([~]1,000 s-1) is 10x faster than bacterial. OXPHOS complexes (ATP synthase, cytochrome c oxidase) show remarkably conserved rates across kingdoms. We interpret these patterns through the lens of metabolic demand, regulatory complexity, and the "rapid bursts and slow declines" framework of enzyme evolution, arguing that context-specific selection pressures--not organismal complexity per se--determine enzyme kinetic tuning.