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Regulation of threonine biosynthesis in barley seedlings (Hordeum vulgare L.)


Homoserine kinase was purified 700-fold by fractional ammonium sulfate precipitation, heat treatment, CM-Sephadex C-50 and DEAE-Sephadex A-50 ion exchange chromatography, and Sephadex G-100 gel filtration. The reaction products O-phosphohomoserine and ADP were the only compounds which caused considerable inhibition of homoserine kinase activity. Product inhibition studies showed non-competitive inhibition between ATP and O-phosphohomoserine and between homoserine and O-phosphohomoserine, and competitive inhibition between ATP and ADP. ADP showed non-competitive inhibition versus homoserine at suboptimal concentrations of ATP. At saturating concentrations of ATP no effect of ADP was observed. The homoserine kinase activity was negligible in the absence of K+ and the Km value for K+ was observed to be 4.3 mmol l−1. A non-competitive pattern was observed with respect to the substrates homoserine and ATP. Threonine synthase in the first green leaf of 6-day-old barley seedlings was partially purified 15-fold by ammonium sulfate fractionation and Sephadex G-100 gel chromatography. Threonine synthase was shown to require pyridoxal 5′-phosphate as coenzyme for optimum activity and the enzyme was strongly activated by S-adenosyl-L-methionine. The optimum pH for threonine synthase activity was 7 to 8.

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Pyridoxal 5′-phosphate






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Aarnes, H. Regulation of threonine biosynthesis in barley seedlings (Hordeum vulgare L.). Planta 140, 185–192 (1978).

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Key words

  • Homoserine kinase
  • Hordeum
  • Threonine synthese