Branched pathways

Branched pathways, also known as branch points (not to be confused with the mathematical branch point), are a common pattern found in metabolism. This is where an intermediate species is chemically made or transformed by multiple enzymatic processes. linear pathways only have one enzymatic reaction producing a species and one enzymatic reaction consuming the species.

Branched pathways are present in numerous metabolic reactions, including glycolysis, the synthesis of lysine, glutamine, and penicillin,^[1] and in the production of the aromatic amino acids.^[2]

In general, a single branch may have ${\displaystyle b}$ producing branches and ${\displaystyle d}$ consuming branches. If the intermediate at the branch point is given by ${\displaystyle s_{i}}$, then the rate of change of ${\displaystyle s_{i}}$ is given by:

${\displaystyle \sum _{i=1}^{b}v_{i}-\sum _{j=1}^{d}v_{j}={\frac {ds_{i}}{dt}}}$

At steady-state when ${\displaystyle ds_{i}/dt=0}$ the consumption and production rates must be equal:

${\displaystyle \sum _{i=1}^{b}v_{i}=\sum _{j=1}^{d}v_{j}}$

Biochemical pathways can be investigated by computer simulation or by looking at the sensitivities, i.e. control coefficients for flux and species concentrations using metabolic control analysis.