Kinetic Properties of Four Enzymes Involved in Proline Metabolism in Cold-acclimated Wheat

Abstract

Two varieties of wheat (Triticum aestivum L.) a winter (Kharkov) and a spring (Glenlea), were acclimated under controlled conditions at 5 °C and 25 °C (12 h photoperiod). Kinetic properties (Km, FmaJKm ratio and Qxo as a function of reduction of substrate concentration) were investigated for enzymatic systems involved in two pathways of proline metabolism: the glutamic acid and the ornithine pathways. Four enzymes were studied, namely proline dehydrogenase (PDH, EC 1.5.1.2), glutamate dehydrogenase (GDH, EC 1.4.1.2-4), glutamine synthetase (GS, EC 6.3.1.2) and ornithine transaminase (OT, EC 2.6.1.13). Kinetic properties of these four enzymes proved to be modulated by cold acclimation, especially in Kharkov, the winter cultivar, which accumulates proline. Firstly, the synthesis of precursors of proline may be augmented and the degradation of proline lessened by either decreasing the Km values of OT or increasing the Km values of PDH. Secondly, the catalytic efficiency ( VmaJKm ratio) of GDH, GS, and OT is increased. Thirdly, the lower values of Ql0 indicate a high capacity of reaction of GS and OT.