= Metabolic engineering
<Metabolic engineering> deliberately modifies a biological system’s reaction pathways and regulation to change its chemical production or other metabolic behavior. Altering an <enzyme>, its expression or the availability of a precursor can redirect <metabolic flux>. The desired outcome is a measured cellular or process phenotype, not merely a changed gene sequence.
A <stoichiometric matrix> provides one useful bookkeeping model. If $\mathbf x$ contains intracellular metabolite amounts per volume, $S$ encodes reaction stoichiometry and $\mathbf v$ contains reaction rates, a simplified balance is
$$
\frac{d\mathbf x}{dt}=S\mathbf v.
$$
A steady intracellular state imposes $S\mathbf v=0$. This condition conserves the represented species but generally does not determine a unique flux vector. Enzyme capacities, substrate supply and other biological constraints are additional information.
An original two-branch example consumes a common precursor through product and competing routes. At steady state, $v_{\mathrm{in}}=v_P+v_B$. Reducing the capacity of the competing route need not increase $v_P$ by the same amount: precursor supply can fall, another route can increase, or the precursor can accumulate until regulation changes. A pathway diagram by itself cannot establish a production gain.
A concrete primary experiment is the 2016 work by <Chan Woo Song>, <Je Woong Kim>, <In Jin Cho> and <Sang Yup Lee>. Engineered <Escherichia coli> strains were screened and modified to produce <3-hydroxypropionic acid> or <malonic acid>; the abstract reports $31.1\,\mathrm{g/L}$ and $3.60\,\mathrm{g/L}$ respectively in fed-batch cultures. The two results concern different engineered product routes, rather than simultaneous outputs guaranteed for every strain.<Metabolic Engineering of Escherichia coli for the Production of 3-Hydroxypropionic Acid and Malonic Acid through β-Alanine Route by Chan Woo Song and others>[Abstract, pathway engineering, enzyme screening and fed-batch results]{ref}
Evaluation therefore compares controlled strains, product identity, accumulated amount, substrate consumption and time. A higher final product concentration does not alone prove a higher yield or production rate. <Metabolic engineering> connects molecular changes with reaction-network balances and <chemical engineering> measurements of the actual production process.
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