ATP coupling coefficients

The existence of two reaction pathways linking glucose and CO2 that differ in ATP stoichiometry and hence in equilibrium constant is by no means a unique situation. Nearly every metabolic pathway has a large in the forward direction but coexists with an oppositely directed pathway, for which in the opposite direction is large. Metabolic sequences are typically strictly unidirectional, and functional reversal of a metabolic conversion nearly always involves a different reaction pathway with a different ATP coupling coefficient. The biosynthesis and degradation of fatty acids supplies another illustration of this generalization. The conversion of 8 /mmoles of aeetyl-SCoA to 1 /imole of palmityl-SCoA requires 14 /imoles of TPNH, or 56 ATP... 

The matrix of the phenomenological coefficients must be positive definite for example, for a two-flow system, we have L0 > 0, Ip >0, and Z/.p Z,pZpo > 0.1,0 shows the influence of substrate availability on oxygen consumption (flow), and Ip is the feedback of the phosphate potential on ATP production (flow). The cross-coupling coefficient Iop shows the phosphate influence on oxygen flow, while Zpo shows the substrate dependency of ATP production. Experiments show that Onsagers s reciprocal relations hold for oxidative phosphorylation, and we have Iop = Zpo. 

Here,, is the mobility coefficient, while L22 is a generalized mobility relating ATP consumption and the chemical potential difference, and /., 2 and L2] are the mechano-chemical coupling coefficients. A given motor/filament system can work in different regimes, and in a regime where the work is performed by the motor, efficiency is defined by... 

The number of ATP s or ATP equivalents generated by a catabolic sequence, or the number of ATP s or ATP equivalents consumed by a biosynthetic sequence, will be termed the coupling coefficient of the sequence. The importance of evolved coupling coefficients may be illustrated by comparison of the two major metabolic processes that link carbohydrate and CO2—photosynthesis and oxidation. 

Metabolic sequences must interact in a very wide variety of ways. Two types seem likely to be of special importance to metabolic regulation (1) interaction between R sequences (with positive coupling coefficients) and U sequences (with negative coefficients) of the type illustrated by Fig. 1, and (2) competition between sequences, either for a branch-point metabolite or for ATP. 

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