Truncated aspartate aminotransferase from alkalophilic Bacillus circulans with deletion of N-terminal 32 amino acids is a non-functional monomer in a partially structured state

: Kravchuk Z, Tsybovsky Y, Koivulehto M, Vlasov A, Chumanevich A, Battchikova N, Martsev S, Korpela T

Publisher: OXFORD UNIV PRESS

: 2001

: Protein Engineering

: PROTEIN ENGINEERING

: PROTEIN ENG

: 14

: 4

: 279

: 285

: 7

: 0269-2139

DOI: https://doi.org/10.1093/protein/14.4.279

Aspartate aminotransferase (AspAT) from alkalophilic Bacillus circulans contains an additional N-terminal sequence of 32 amino acid residues that are absent in all other AspATs from different sources, Modeling suggested that this sequence forms two alpha -helical segments which establish a continuous network of interactions on the surface of the molecule. In the present study, we studied the role of the N-terminal sequence in folding and stability of AspAT by applying the scanning calorimetry, and CD and fluorescence spectroscopies to the native and truncated enzymes. Truncated AspAT (Delta2 alpha mutant) devoid of N-terminal residues cannot provide sufficient potential of quaternary intersubunit and subunit-cofactor interactions, which results in a monomeric non-functional conformation. However, the residual tertiary interactions in the Delta2 alpha mutant are sufficient to: i) provide stability of a residual structure over a wide pH range; ii) confer moderate cooperativity of the denaturant-induced transition while only low cooperativity of the thermal transition, and iii) maintain the hydrophobic core of a part of the structure which prevents aromatic fluorophores from quenching by water, Furthermore,the present study provides evidence that AspAT from the alkalophilic bacterium follows unfolding pathway comprising a stable non-functional intermediate, in contrast to a two-state mechanism of the thermophilic AspAT from Sulfolobus solfataricus.