Structures of beta-ketoacyl-acyl carrier protein synthase I complexed with fatty acids elucidate its catalytic machinery.
BACKGROUND
beta-ketoacyl-acyl carrier protein synthase (KAS) I is vital for the construction of the unsaturated fatty acid carbon skeletons characterizing E. coli membrane lipids. The new carbon-carbon bonds are created by KAS I in a Claisen condensation performed in a three-step enzymatic reaction. KAS I belongs to the thiolase fold enzymes, of which structures are known for five other enzymes.


RESULTS
Structures of the catalytic Cys-Ser KAS I mutant with covalently bound C10 and C12 acyl substrates have been determined to 2.40 and 1.85 A resolution, respectively. The KAS I dimer is not changed by the formation of the complexes but reveals an asymmetric binding of the two substrates bound to the dimer. A detailed model is proposed for the catalysis of KAS I. Of the two histidines required for decarboxylation, one donates a hydrogen bond to the malonyl thioester oxo group, and the other abstracts a proton from the leaving group.


CONCLUSIONS
The same mechanism is proposed for KAS II, which also has a Cys-His-His active site triad. Comparison to the active site architectures of other thiolase fold enzymes carrying out a decarboxylation step suggests that chalcone synthase and KAS III with Cys-His-Asn triads use another mechanism in which both the histidine and the asparagine interact with the thioester oxo group. The acyl binding pockets of KAS I and KAS II are so similar that they alone cannot provide the basis for their differences in substrate specificity.
Olsen
J G
JG
Centre for Crystallographic Studies, University of Copenhagen, DK-2100, Denmark.
Kadziola
A
A
von Wettstein-Knowles
P
P
Siggaard-Andersen
M
M
Larsen
S
S
D015099:3-Oxoacyl-(Acyl-Carrier-Protein) Synthase / Q000737:chemistry*; D002384:Catalysis; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D003653:Decarboxylation; D004926:Escherichia coli / Q000737:chemistry; D064429:Fatty Acid Synthases / Q000737:chemistry; D005227:Fatty Acids / Q000737:chemistry*; D007527:Isoenzymes / Q000737:chemistry*; D008958:Models, Molecular
D005227:Fatty Acids; D007527:Isoenzymes; C078182:beta-ketoacyl-acyl carrier protein synthase I; C077402:3-ketoacyl-acyl carrier protein synthase III; D015099:3-Oxoacyl-(Acyl-Carrier-Protein) Synthase; D064429:Fatty Acid Synthases