My enzyme is a monomer but sometimes exists as homo-oligomer. The kinetic plot of the enzyme was hyperbolic. The current project on the enzyme involves incubating the enzyme with some compounds at 30oC for 1 h before initiation the reaction. Interestingly, the enzyme exhibited sigmoidal kinetic curve after the 1 h incubation. Although, few articles reported sigmoidal for the enzyme.
My major puzzle is, the later kinetic (with the incubation) study has a two-fold Vmax of the former (without the incubation), yet all reaction conditions, besides the incubation, are the same.
Decomposition of the homo-oligomer is the obvious first choice. It may occur from slight difference of temperature, water content, or pH, also from loss of the chain initiator or terminator, and by an equilibrium shift from increase of receptive sites.
Is your enzyme a protein kinase that is capable of activation by autophosphorylation? This would result in sigmoidal kinetics as long as the enzyme was not fully phosphorylated at the start.
Another possibility is that the enzyme is allosterically activated by one of its products (which would be unusual).
A third reason for the observation could be that the enzyme suffers from strong substrate inhibition, which is relieved as the substrate is consumed, causing the reaction rate to increase.
may be your enzyme is allosterically activated by the substrate (homotropic effect)
Your enzyme might be a morpheein, an enzyme that morphs (not to be mixed up with morphine, a drug that induces sleep). Morpheeins exist in two stable conformations, that form different homo-oligomers with different activity. The most well-known example is PBG-synthase in haem synthesis. Note that sigmoidal kinetics is a sign that the enzyme is not only a homo-oligomer, but that the subunits influence each others.
Vmax can also be changed by post-translational modification (e.g., (de-)phosphorylation, (de-)methylation), the loss if an inhibitor or inactivator, or the presence of an allosteric activator.
Vmax is NOT a constant. it is dependent on enzyme concentration. But as Ben Amar Cheba told it can be a cooperativ enyzme activated by homotropic activator=substrate itself. Allostery is another phenomenon if the non-substrate effector =heterotopic effector influences the activity. allosteric enzyme MUST exhibit cooperativity in substrate binding or homotropic allostery. Two types of allosteric enyzmes are (see atachment).
Thank you, Deker, Shapiro, Cheba, Buxbaun and Ewa. I appreciate your answers and suggestions.
Kind regards.
positive effectors (allosteric activators)lead to an increase in enzyme Vmax
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