TABLE 9.

Differences between biologically mediated reaction of cellulose and soluble carbohydrate substrates

Soluble carbohydratesCellulose
Enzymatic reaction mechanism and kinetics
    Essentially all potential substrate-reactive sites are accessibleOnly a small fraction of substrate-reactive sites are accessible
    Substrate commonly in excess; hence, rate is proportional to [ET]Excess substrate is uncommon; hence, at least partial rate saturation with increasing [ET] is usually encountered
    Concentration is the only kinetically important substrate state variableConversion is an additional kinetically important substrate state variable in at least some systems
    r/[CE] is a constant throughout the course of an irreversible reactionr/[CE] decreases sharply over the course of the reaction
    km, the substrate concentration at which half the maximum rate is observed, is a constant independent of [ET]The substrate concentration at which half the maximum rate is observed increases with increasing [ET]
Cell growth, observable in batch culture
    Exponential growth, during which substrate is in excess and the concentration of cells is rate limiting, is readily observedExponential growth is typically not observed because substrate is typically not in excess and perhaps also because of declining substrate reactivity
    Allocation of substrate between catabolism and cell synthesis is a key metabolic choice facing the cellIn addition to allocation between catabolism and anabolism, a second key metabolic choice involves allocation of carbon and energy between synthesis of cells and cellulase
    ATP is synthesized via glycolysis and post-pyruvate metabolism (anaerobes)Phosphorolytic cleavage of cellodextrins and cellobiose provides an additional potential route for synthesizing high-energy bonds
Cell growth, chemostat culture
    Substrate reactivity is equal to that leaving the fermentorSubstrate reactivity is equal to the weighted sum of particle reactivities integrated over the time each particle (or fraction of particles) spends in the fermentor
    Steady-state substrate concentration is independent of the feed substrate concentrationSteady-state substrate concentration increases with increasing feed substrate concentration