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The final substrate-level phosphorylation in glycolysis provides a useful experimental handle: pyruvate kinase transfers a phosphate group from phosphoenolpyruvate to ADP, producing pyruvate and ATP. Adding lactate dehydrogenase couples each pyruvate formed to consumption of one NADH:These equations select free ionic forms consistently: phosphoenolpyruvate has charge $-3$, ADP $-3$, ATP $-4$, and pyruvate and lactate each $-1$; NAD+ has one more positive charge than NADH. Each line conserves charge. Cancelling pyruvate gives one NADH consumed per ATP formed in this coupled system. A whole-pathway NADH measurement has a different meaning, because the earlier glycolysis redox step produces NADH.

The published An Optimized Enzyme-Coupled Spectrophotometric Method for Measuring Pyruvate Kinase Kinetics reports recombinant pyruvate kinase prepared in Escherichia coli, purified by chromatography and size-exclusion chromatography. UV-transparent microplate wells contained ADP, phosphoenolpyruvate, NADH, lactate dehydrogenase and a buffer solution with magnesium ions. An Epoch plate reader used spectrophotometry to follow absorbance at $340\mathrm{nm}$ every five to ten seconds for one minute at $25{}^{\circ }\mathrm{C}$. Its Figure 2 presents $A_0-A_t$, increasing as NADH is consumed. Heat-inactivated pyruvate kinase gave minimal background; a validated active preparation was the positive control. These comparisons support an enzyme-dependent readout in the prepared system, rather than a whole-biological cell glycolysis measurement.^[ref]

The same report uses $0.5\mathrm{cm}$ for optical path length in Procedure B.2.i, but approximately $0.3\mathrm{cm}$ in Data analysis §3. An absolute conversion requires the actual path length and sufficient coupling enzyme activity; no measured ATP rate is inferred here.^[ref]

For an original dimensional example, stipulate a blank-corrected absorbance decline $-d{A}_{340}/dt=0.024\mathrm{mi}{\mathrm{n}}^{-1}$, a calibrated path length $\ell =0.40\mathrm{cm}$ and a chosen calibration coefficient $\epsilon =6000\mathrm{L}\mathrm{mo}{\mathrm{l}}^{-1}\mathrm{c}{\mathrm{m}}^{-1}$. These are invented teaching inputs, not the protocol's measurements or its quoted coefficient. The Beer-Lambert law givesIn a stipulated $0.20\mathrm{mL}$ reaction volume, this is $2\mathrm{nmol}\mathrm{mi}{\mathrm{n}}^{-1}$ of NADH consumed and, conditionally, $2\mathrm{nmol}\mathrm{mi}{\mathrm{n}}^{-1}$ of ATP formed. The conditional inference requires rapid pyruvate consumption by the coupling enzyme, no other NADH-consuming chemical reactions, and a linear, correctly calibrated optical response. The readout measures this last glycolysis step; it cannot establish the full two-net-ATP-per-glucose balance by itself.

Phosphoenolpyruvate supplies a phosphate group to ADP in the final glycolysis step catalyzed by pyruvate kinase. The free ionic form used in the worked balance has charge $-3$.

Pyruvate kinase catalyzes phosphate transfer from phosphoenolpyruvate to ADP, producing pyruvate and ATP in glycolysis.

Saurabh Upadhyay authored an enzyme-coupled spectrophotometric method for measuring pyruvate kinase kinetics. A coupled reaction converts product formation into a change in an optical reporter.

The reporter measures the intended enzyme rate only when coupling is sufficiently rapid and the signal is converted with an appropriate optical path length. This separates an observed absorbance slope from an absolute chemical rate.^[ref]

Identity source: pubmed.ncbi.nlm.nih.gov/40873479/

Substrate-level phosphorylation forms ATP by enzyme-catalyzed transfer of a phosphate group from a phosphorylated substrate to ADP. In canonical glycolysis, two such transfers occur for each three-carbon route, giving four gross ATP per glucose. Unlike oxidative phosphorylation, these transfers do not require an electron transport chain.^[ref]

The pyruvate kinase example under Coupled-enzyme readout of glycolytic ATP formation separates the material transfer from its detection. A favourable coupled chemical reaction supplies the driving change in Gibbs free energy; the enzyme supplies a catalytic route. Counting ATP-forming transfers gives a gross yield, while subtracting upstream ATP use gives the net yield.