Methods for the economical production of biofuel from biomass

A method of making isobutanol, the method comprising:

• a) providing a feedstock comprising a sugar selected from the group consisting of:
• i) a six-carbon sugar;
• ii) a six-carbon sugar oligomer;
• iii) a five-carbon sugar;
• and
• iv) any combination of i) through iii);
• b) contacting the feedstock of a) with an engineered microorganism under suitable fermentation conditions such that said engineered microorganism converts the feedstock into isobutanol at:
• i) a yield of at least 68 percent theoretical yield;
• ii) a productivity of at least 0.75 grams isobutanol per liter per hour;
• and
• iii) a total titer of at least 1.6% (w/v) of isobutanol in water, wherein said engineered microorganism expresses exogenous genes encoding an acetolactate synthase that catalyzes the conversion of pyruvate to acetolactate, a ketol-acid reductoisomerase that catalyzes the conversion of acetolactate to 2,3-dihydroxyisovalerate, a dihydroxy acid dehydratase that catalyzes the conversion of 2,3-dihydroxyisovalerate to α-ketoisovalerate, a ketoisovalerate decarboxylase that catalyzes the conversion of α-ketoisovalerate to isobutyraldehyde, and an isobutyraldehyde dehydrogenase that catalyzes the conversion of isobutyraldehyde to isobutanol;
• and
• c) recovering the isobutanol.