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Originally published In Press as doi:10.1074/jbc.M100329200 on March 15, 2001
J. Biol. Chem., Vol. 276, Issue 22, 19141-19149, June 1, 2001
Circular Permutation of 5-Aminolevulinate Synthase
MAPPING THE POLYPEPTIDE CHAIN TO ITS FUNCTION*
Anton V.
Cheltsov §,
Michael J.
Barber ¶, and
Gloria C.
Ferreira ¶ **
From the Department of Biochemistry and Molecular
Biology, College of Medicine, ¶ Institute for Biomolecular
Science, and H. Lee Moffitt Cancer Center and Research
Institute, University of South Florida, Tampa, Florida 33612
5-Aminolevulinate synthase is the first
enzyme of the heme biosynthetic pathway in non-plant eukaryotes and
some prokaryotes. The enzyme functions as a homodimer and requires
pyridoxal 5'-phosphate as a cofactor. Although the roles of defined
amino acids in the active site and catalytic mechanism have been
recently explored using site-directed mutagenesis, much less is known
about the role of the 5-aminolevulinate synthase polypeptide chain
arrangement in folding, structure, and ultimately, function. To assess
the importance of the continuity of the polypeptide chain, circularly permuted 5-aminolevulinate synthase variants were constructed through
either rational design or screening of an engineered random library.
One percent of the random library clones were active, and a total of 21 active variants had sequences different from that of the wild type
5-aminolevulinate synthase. Out of these 21 variants, 9 displayed
unique circular permutations of the 5-aminolevulinate synthase
polypeptide chain. The new termini of the active variants disrupted
secondary structure elements and loop regions and fell in 100 amino
acid regions from each terminus. This indicates that the natural
continuity of the 5-aminolevulinate synthase polypeptide chain and the
sequential arrangement of the secondary structure elements are not
requirements for proper folding, binding of the cofactor, or assembly
of the two subunits. Furthermore, the order of two identified
functional elements (i.e. the catalytic and the
glycine-binding domains) is apparently irrelevant for proper functioning of the enzyme. Although the wild type 5-aminolevulinate synthase and the circularly permuted variants appear to have similar, predicted overall tertiary structures, they exhibit differences in the
arrangement of the secondary structure elements and in the
cofactor-binding site environment. Taken together, the data lead us to
propose that the 5-aminolevulinate synthase overall structure can be
reached through multiple or alternative folding pathways.
*
This work was supported in part by National Institutes of
Health Grant DK52053 (to G. C. F.).The costs of publication of this article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
§
Recipient of American Heart Association/Florida Division
Predoctoral Fellowship 9850021FL.
**
To whom correspondence should be addressed: Dept. of Biochemistry
and Molecular Biology, College of Medicine, University of South
Florida, 12901 Bruce B. Downs Blvd., Tampa, FL 33612. Tel.: 813-974-5797; Fax: 813-974-0504; E-mail: gferreir@hsc.usf.edu.
Copyright © 2001 by The American Society for Biochemistry and Molecular Biology, Inc.

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Copyright © 2001 by the American Society for Biochemistry and Molecular Biology.
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