About this Journal Submit a Manuscript Table of Contents
BioMed Research International
Volume 2013 (2013), Article ID 353270, 10 pages
http://dx.doi.org/10.1155/2013/353270
Research Article

Asymmetry of the Active Site Loop Conformation between Subunits of Glutamate-1-semialdehyde Aminomutase in Solution

1Dipartimento di Farmacia, Università di Parma, Parco Area delle Scienze 23/A, 43124 Parma, Italy
2Dipartimento di Neuroscienze, Università di Parma, Via Volturno 39, 43125 Parma, Italy
3Istituto Nazionale di Biostrutture e Biosistemi, Viale Medaglie d’Oro 305, 00136 Roma, Italy
4Dipartimento di Scienze Biochimiche “A. Rossi-Fanelli”, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy

Received 15 May 2013; Accepted 27 June 2013

Academic Editor: Barbara Cellini

Copyright © 2013 Barbara Campanini et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Glutamate-1-semialdehyde aminomutase (GSAM) is a dimeric, pyridoxal 5′-phosphate (PLP)- dependent enzyme catalysing in plants and some bacteria the isomerization of L-glutamate-1-semialdehyde to 5-aminolevulinate, a common precursor of chlorophyll, haem, coenzyme B12, and other tetrapyrrolic compounds. During the catalytic cycle, the coenzyme undergoes conversion from pyridoxamine 5′-phosphate (PMP) to PLP. The entrance of the catalytic site is protected by a loop that is believed to switch from an open to a closed conformation during catalysis. Crystallographic studies indicated that the structure of the mobile loop is related to the form of the cofactor bound to the active site, allowing for asymmetry within the dimer. Since no information on structural and functional asymmetry of the enzyme in solution is available in the literature, we investigated the active site accessibility by determining the cofactor fluorescence quenching of PMP- and PLP-GSAM forms. PLP-GSAM is partially quenched by potassium iodide, suggesting that at least one catalytic site is accessible to the anionic quencher and therefore confirming the asymmetry observed in the crystal structure. Iodide induces release of the cofactor from PMP-GSAM, apparently from only one catalytic site, therefore suggesting an asymmetry also in this form of the enzyme in solution, in contrast with the crystallographic data.