Characterization of carrot pectin methylesterase

The most alkaline form of pectin methylesterase was purified from ripe carrot roots and used for structural analysis. Determination of an N-terminal blocking group and of the primary structure allowed comparisons with other forms, and facilitated crystallographic determination of the three-dimensional structure. The mature enzyme has 319 residues and the N-terminal blocking group was shown to be a pyroglutamyl residue derived from a glutaminyl cyclization. Few other methylesterases have been isolated and assigned to exact mature forms, and together with the present enzyme, only two have been analyzed in three-dimensional structure. However, comparison of 39 forms, mainly from GenBank data, reveals clear relationships and identifies subgroups of this enzyme type, deviating in structure but centering around two functionally important and conserved Asp residues at positions 136 and 157 in the carrot enzyme.