代謝回転数と触媒効率

Turnover number, or kcat, indicates how rapidly an enzyme converts substrate molecules into products. It represents the maximum number of substrate molecules transformed per unit time at each enzyme active site. Turnover numbers vary dramatically across enzymes, ranging from fewer than one substrate molecule to millions of molecules per second, reflecting vast differences in catalytic speed.

Catalytic efficiency is calculated as the ratio of kcat to KM. An enzyme with high kcat quickly catalyzes substrate transformation, while low KM indicates strong substrate binding. Enzymes with larger kcat to KM ratios are more efficient. This metric reveals how effectively an enzyme accelerates a biochemical reaction by combining both catalytic speed and substrate affinity.

Catalase has the highest known turnover number, converting up to 2.8×10⁶ molecules of hydrogen peroxide per second into water and oxygen. This extraordinary speed reflects its specialized role in rapidly neutralizing toxic hydrogen peroxide in cells. Most enzymes have turnover numbers between 1 and 1,000 molecules per second, making catalase's rate millions of times faster than typical enzymes.

No. High turnover number alone does not guarantee high catalytic efficiency. An enzyme must also have low KM, meaning strong substrate affinity, to be truly efficient. RuBisCO exemplifies this: it has a very low turnover number of only 3 CO₂ molecules per second, yet its abundance in leaves compensates for its slow catalytic rate.

Most enzymes have average catalytic efficiency around 10⁵ M⁻¹s⁻¹, making them moderately efficient. Catalytically perfect enzymes achieve efficiency between 10⁸-10⁹ M⁻¹s⁻¹, representing superefficiency. These rare enzymes operate at near-diffusion-limited rates, meaning they catalyze reactions as fast as substrate molecules can physically encounter their active sites.

Chymotrypsin, a pancreatic enzyme, has a turnover number of 100 molecules per second. Without enzymatic catalysis, peptide bonds would take hundreds of years to break in water at neutral pH. This high turnover number enables chymotrypsin to rapidly digest proteins in the intestine, demonstrating how catalytic speed directly impacts physiological function.

When an enzyme binds multiple substrates, its catalytic efficiency for each substrate depends on the kcat to KM ratio. The substrate with the highest kcat to KM ratio is processed most efficiently. This ratio integrates both binding affinity (KM) and catalytic speed (kcat), allowing enzymes to preferentially catalyze reactions with their most favorable substrates.