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The type II, or dissociated, fatty acid synthase system is the primary source for membrane fatty acids in bacteria and plants. This ubiquitous and highly conserved pathway has been most extensively studied in Escherichia coli and consists of a collection of individual enzymes that are encoded by separate genes. This general organization contrasts sharply with that of the type I fatty acid synthase system in animals in which a single multifunctional polypeptide catalyzes all of the reactions in the elongation pathway. This distinguishing characteristic makes the type II system a prime target for antibacterial agents. In the type II system, fatty acid elongation occurs in two-carbon steps by the Claisen condensation of malonyl-ACP with acyl-ACP (ACP is acyl carrier protein, a small, acidic, soluble protein that shuttles the elongating chain between enzymes). Three enzymes (FabB, FabF, and FabH) catalyze these condensation reactions, and a number of other enzymes perform additional necessary reactions within the pathway. Dr. Charles O. Rock in the Department of Biochemistry at St. Jude has studied the type II system for many years, and our groups are collaborating to solve the crystal structures of these enzymes.
Recently, we completed the 1.8Å structure of FabH, and crystals of other enzymes are becoming available. FabH is the initial enzyme in the pathway, and it mediates the condensation of acetyl coenzyme A and malonyl-ACP to form b-ketoacyl-ACP. The structure shows the active site in exquisite detail, and the enzyme's mechanism has been dissected with the help of several site-directed mutants. The active site is at the bottom of a deep hydrophobic tunnel that contains a molecule of coenzyme A. The adenine and phosphate moieties of coenzyme A appear to interact specifically with conserved amino acids around the hole entrance. A notable feature of the FabH structure is that it displays an internal dyad symmetry indicative of gene duplication. This genetic event may have been crucial to the evolution of the two-step enzyme mechanism.
Last update: April 2003