Pathway Summary

Nopaline and octopine are opines derived from arginine. They are biosynthesized in the cells of plant crown gall tumors. These tumors are caused by infection with the plant pathogen Agrobacterium tumefaciens. The enzymes for both the biosynthesis and degradation/utilization of opines are encoded by the tumor-inducing Ti plasmids found in A. tumefaciens. This host-pathogen interaction has an interesting mechanism. A. tumefaciens infects wounded plants in response to chemical signals released by plant cells. Infection by this organism involves products of the vir genes encoded by a Ti plasmid. These gene products cause the transfer of oncogenic fragments of DNA (T-DNA) from the Ti plasmid into the nucleus of host plant cells, where they become integrated into the host genome.

In plant cells, the T-DNA onc genes express plant hormones causing neoplastic growth. Genes encoding opine synthases (ops genes such as nos encoding nopaline synthase and ocs encoding octopine synthase) are also expressed and direct the biosynthesis of opines (EC 1.5.1.11 and 1.5.1.19 reactions in the reverse direction). These synthases are soluble, NAD(P)H-dependent dehydrogenases [Zanker94] (see MetaCyc pathway pyruvate-derived opine biosynthesis II (marine invertebrates)). Opines produced in the plant tumor cells are secreted into the environment, enter the bacteria via opine permeases, and induce their catabolism by opine-specific enzymes. Opines therefore provide a source of carbon, nitrogen and energy for A. tumefaciens inside the plant tumor cells (in [Cho05, Kalogeraki98, Hong97], and reviewed in [Zhu00]).

The proteins required for uptake and catabolism of nopaline and octopine are organized into operons in the noc region of a nopaline-type Ti plasmid, or the occ region of an octopine-type Ti plasmid [Akakura02, Kalogeraki98, vonLintig94]. The enzymatic degradation of nopaline to arginine and α-ketoglutarate by nopaline oxidase is shown here. Nopaline oxidase is a membrane-bound opine oxidase that does not require an added cofactor when assayed in crude preparations. However, it was postulated by sequence analysis to require a cofactor such as FAD, FMN, or NAD(P)H. Therefore, the electron acceptor(s) for this reaction remains to be identified in this organism [Zanker94]. Clicking on the pathway link shows the conversion of arginine to proline and then glutamate in A. tumefaciens [Zanker94, Cho97, Cho96]. A similar degradation of octopine to arginine and pyruvate by octopine oxidase in this organism is shown in MetaCyc pathway octopine degradation.