The Concept of Metabolic Pathway - Part 1
Introduction
Metabolism generally occurs by orderly, step-wise, sequence that are dependent on the basic laws of thermodynamics. These metabolic pathways consists of a series of progressive, individual chemical reactions.
A Single Step
A single step involves the conversion of a precursor (A) to a given product (B). This reaction is catalysed by a protein molecule, Ea, called an enzyme. An enzymatic reaction is reversible, the forward reaction is described by a rate constant, Kf, whereas the reverse reaction is described by Kr.
Numerous Step
Such a sequence may continue for numerous individual steps, so that the original substrate, A, is converted to the ultimate product, P. This is a typical liner metabolic pathway.
Metabolic Pathways may begin as a linear sequence of steps and the branch into 2 or more separate pathways. In this way, the precursor or nutrient may lead to the production of several products, usually the first step beyond the common metabolite, B, is called the committed step in the production of P'.
Handling Reversible Reactions
To achieve the necessary control of the metabolism, It is desirable for a given step to be irreversible. although all enzymatic reactions are in principle reversible, in some steps the change in free energy may be sufficiently large as to make the reverse reaction improbable. Furthermore , the concentration of product is too low for the reverse reaction to be appreciable. Under these circumstances the reaction is sad to be physiologically irreversible.
For the metabolic pathway to proceed from B = A, a second process catalysed by another enzyme, Ea', is required. The 2 enzyme systems, Ea and Ea'. This prevents Ea from catalysing the conversion of A=B, only to have B restored to A by Ea'. Such a process is called futile cycle and would involve a wasteful consumption of energy.
Independent Control
Some potential futile cycles may be viewed as the proceeding from a precursor to a product by way of a number of intervening steps and then back to the precursor by an entirely different series. The enzymes that catalyse precursor = product are controlled independently of those leading in the reverse direction. Furthermore, the 2 sequences may occur simultaneously, the net achievement in either direction being dependent on the metabolic needs.