The unique enzymatic function of field bean (Dolichos lablab) d-galactose specific lectin: a polyphenol oxidase

Abstract

The polyphenol oxidase (PPO) of field bean (Dolichos lablab) is a tetramer made up of two subunits of mass 29,000 and 31,000 Da. The amino acid sequence of the tryptic peptides showed approximately 90% sequence identity to the D: -galactose specific legume lectins. The haemagglutinating activity of a pure and homogenous preparation of PPO measured using human erythrocytes was 1261 HAU mg(-1) protein and was inhibited by D: -galactose. Purification by galactose-sepharose chromatography also indicated that the PPO and haemagglutinating activities were associated with a single protein. Crude extracts of other legumes did not exhibit PPO activity, yet cross reacted with anti-PPO antibodies. This dual function protein with PPO and haemagglutinating activity is unique to field bean. The two activities are independent of each other occurring at different loci on the protein. These observations further evidence and strengthen the assumption that galactose specific legume lectins have enzymatic function. Both PPO and lectins are proteins that play a vital role in the defense mechanism of plants. The complementarity of these two simultaneous and independent powerful defense mechanisms exhibited by a single protein renders it a candidate gene for the development of inbuilt plant protection.

Structural basis of noncanonical polyphenol oxidase activity in DLL‐II: A lectin from Dolichos lablab

“…Dolichos lablab , commonly known as field bean, is a leguminous plant largely grown in India. Field bean seeds contain one glucose/mannose‐specific lectin (DLL‐I) and a galactose/N‐acetyl‐ d ‐galactosamine (GalNAc)‐specific lectin (DLL‐II) . DLL‐II is a tetrameric protein with two subunits of masses 29 and 31 kDa, respectively.…”

“…Previously, Kanade et al. [5] characterized DLL‐II and found that unlike the typical lectins, DLL‐II and also the closely related DBL—a lectin from Dolichos biflorus —have relatively weaker binding affinities for their sugar ligands . However, a distinctive moonlighting function of DLL‐II is a high polyphenol oxidase (PPO) activity that oxidizes diphenols to o‐quinones (EC 1.10.3.1) (Fig.…”

“…However, a distinctive moonlighting function of DLL‐II is a high polyphenol oxidase (PPO) activity that oxidizes diphenols to o‐quinones (EC 1.10.3.1) (Fig. ) . A number of lectins showing such bi‐functionality, that is, possessing an enzymatic function apart from their lectin activity, are called leczymes.…”

“…Biochemical studies have demonstrated that the two activities—lectin and PPO—are independent of each other, and in all likelihood, occur at different sites on the same protein . In contrast to the extensively studied sugar binding properties of DLL‐II, its PPO activity, substrate binding, and the associated catalytic mechanism have not been investigated.…”

“…Typically, legume lectins bind two metal ions; a Ca 2+ common to all lectins, including both DBL and DLL‐II, and the second is a Mn 2+ ion. Incidentally, DLL‐II is also the only lectin reported thus far, that binds Cu 2+ instead of a Mn 2+ ion .…”