The lactate dehydrogenase-Etf complex catalyzes the oxidation of lactate to pyruvate. It uses flavin-based electron confurcation to drive endergonic lactate oxidation with NAD(+) as oxidant at the expense of simultaneous exergonic electron flow from reduced ferredoxin to NAD(+). The electron transfer flavoprotein (Etf) mediates the electron transfer between the different donors and acceptors. lactate + 2 NAD(+) + 2 reduced [2Fe-2S]-[ferredoxin] = 2 NADH + 2 oxidized [2Fe-2S]-[ferredoxin] + pyruvate Binds 1 FAD per subunit. Binds 1 AMP per subunit. Activity is stimulated by divalent cations. Highest stimulation is observed with Ca(2+). Optimum pH is 7.0. Optimum temperature is between 20 and 30 degrees Celsius. Part of the stable heterotrimeric lactate dehydrogenase-Etf complex, which is formed by the lactate dehydrogenase LctD and the electron-transferring flavoprotein (Etf) alpha (LctC) and beta (LctB) subunits. Belongs to the ETF beta-subunit/FixA family.