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, and in the production of the aromatic amino acids.

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.