Page 5 of 18
| (continued) p. 322 free energy source an enzyme can only catalyze a reaction towards but not against the equilibrium. If the product dissociation step is thought irreversible, then whether the S to P conversion is energetically favorable or not, the reaction will proceed clockwise anyway. We know from our experience with hundreds of enzymes that this is not the case. The inconsistency can easily be be removed by writing a reversible product dissociation step. Now the reaction becomes scheme 5. Scheme 5 has been used to describe many membrane transport and energy transducing processes. The reaction of scheme 5 can proceed clockwise or counter-clockwise depending on the sign of the free energy of the S to P conversion, clockwise if (delta)G is negative and counter-clockwise if it is positive. More importantly, when a cell membrane is involved this scheme represents an efficient mechanism for energy and signal transductions as we shall see later; yet, the enzyme E is basically a Michaelis-Menten enzyme. An external energy source can be coupled to this scheme so that enzyme can catalyze a reaction against its chemical potential gradient. Scheme 5 has a characteristic frequency. Only an oscillating driving force the frequency of which matches this charateristic frequency will be effective in propelling the "catalytic wheel" of scheme 5, as will be discussed more explicitly in the following sections. |
