Enhanced pyruvate to acetolactate conversion in yeast

1. A recombinant yeast microorganism for producing isobutanol, the recombinant yeast microorganism comprising, an engineered isobutanol biosynthetic pathway, wherein said engineered isobutanol biosynthetic pathway comprises the following substrate to product conversions:

(a) pyruvate to acetolactate;

(b) acetolactate to 2,3-dihydroxy-isovalerate;

(c) 2,3-dihydroxy-isovalerate to α-ketoisovalerate;

(d) α-ketoisovalerate to isobutyryl-CoA; and

(e) isobutyryl-CoA to isobutyraldehyde,

wherein

i) the substrate to product conversion of step (a) is performed by an acetolactate synthase enzyme;

ii) the substrate to product conversion of step (b) is performed by an acetohydroxy acid isomeroreductase enzyme;

iii) the substrate to product conversion of step (c) is performed by an acetohydroxy acid dehydratase enzyme;

iv) the substrate to product conversion of step (d) is performed by a branched-chain keto acid dehydrogenase; and

v) the substrate to product conversion of step (e) is performed by an acylating aldehyde dehydrogenase enzyme

and wherein the recombinant yeast microorganism is substantially free of at least one enzyme having pyruvate decarboxylase activity and, is substantially free of at least one enzyme having glycerol-3-phosphate dehydrogenase activity.

2. The recombinant yeast of claim 1 , wherein the engineered isobutanol biosynthetic pathway further comprises the following pathway step:

(f) isobutyraldehyde to isobutanol

wherein the substrate to product conversion of step (f) is performed by an alcohol dehydrogenase enzyme.

3. The recombinant yeast of claim 1 , wherein the conversion of pyruvate to acetolactate is catalyzed by a cytosol-localized polypeptide having acetolactate synthase activity.

4. The recombinant yeast of claim 3 , wherein the polypeptide having acetolactate synthase activity has a substrate preference for pyruvate over keto butyrate.

5. The recombinant yeast of claim 3 , wherein the polypeptide having acetolactate synthase activity is a Lactococcus lactis acetolactate synthase.

6. The recombinant yeast of claim 3 , wherein the polypeptide having acetolactate synthase activity is a Bacillus subtilis acetolactate synthase.

7. The recornbinant yeast of claim 1 , wherein the yeast cell comprises a disruption in at least one gene encoding a polypeptide having pyruvate decarboxylase activity or controlling pyruvate decarboxylase gene expression.

8. The recombinant yeast of claim 7 , wherein the polypeptide is selected from the group consisting of PDC1, PDC2, PDC5, and PDC6.

9. The recombinant yeast of claim 8 comprising a disruption in PDC1 and PDC5.

10. The recombinant yeast of claim 1 comprising a disruption in at least one gene encoding an NAD-dependent glycerol-3-phosphate dehydrogenase selected from the group consisting of: GPD1 and GPD2.

11. The recombinant yeast of claim 1 , wherein the conversion of pyruvate to acetolactate is at least about 60% of theoretical yield.

12. The recombinant yeast of claim 1 , further comprising a balance in reducing equivalents, wherein the conversion of pyruvate to acetolactate is at least about 86% of theoretical yield.

13. The recombinant yeast of claim 1 , wherein the yeast further comprises a construct for heterologous expression of a secondary alcohol dehydrogenase.

14. The recombinant yeast of claim 1 , wherein the yeast is Kluyveromyces lactis.

15. The recombinant yeast of claim 1 , wherein the yeast is Saccharomyces cerevisiae.

16. A method of producing isobutanol, comprising;

(a) providing the recombinant yeast microorganism according to claim 1 ; and

(b) growing the recombinant yeast microorganism under conditions where isobutanol is produced.

17. The method of claim 16 further comprising (c) recovering the isobutanol.