Metabolic Interactions Between AAAM and the Aspartate Family Pathway

Aspartate, the substrate of AK, serves not only as the precursor for the aspartate family pathway but is also the immediate precursor for the amide amino acid asparagine via the activity of asparagine synthetase (Fig. 3.1). As discussed before, aside from being a building block of proteins, asparagine also possesses several additional important functions in nitrogen assimilation and transport (Lam et al., 1995, 1998). How then is either the metabolic channeling of aspartate into asparagine or the aspartate family amino acids regulated? Molecular analyses suggest that this channeling may be regulated by the expression of genes encoding asparagine synthetase and AK. Plants possess two forms of asparagine synthetase genes. The expression of one is induced by light and sucrose (similar to the gene encoding AK/HSD) to enable asparagine synthesis during the day, while expression of the other is repressed by light and sucrose and is induced during the night (Lam et al., 1995, 1998). Notably, expression of at least one of the Arabidopsis AK/HSD genes is stimulated by light and sucrose in a very similar manner to that of the asparagine synthase gene that is expressed during the daytime (Zhu-Shimoni and Galili, 1998; Zhu-Shimoni et al., 1997). Thus, assuming that other genes of the aspartate family pathway respond to light and sucrose similarly to this AK/HSD gene, one can hypothesize that during the day aspartate is apparently channeled both into asparagine and into the aspartate family pathway to allow synthesis of all of its end-product amino acids. During the night, the aspartate family pathway is relatively inefficient and aspartate channels preferentially into asparagine. Indeed, asparagine levels are much higher, while lysine levels are lower at night than during daytime (Lam et al., 1995).

Channeling of aspartate into the aspartate family pathway may not only be regulated by photosynthesis and ‘‘day/night’’ cycles. An unexpected observation supporting such a possibility was recently reported following the analysis of an Arabidopsis knockout mutant in one of its two DHPS genes (Craciun et al., 2000; Sarrobert et al., 2000). In this mutant, threonine levels increased. However, the extent of the increase (between 10- and 80-fold, depending on growth conditions) far exceeded the slight ~50% reduction in lysine levels, implying that the reduction in DHPS activity triggered an enhanced channeling of aspartate into the threonine branch of the aspartate family pathway (Fig. 3.1). This enhanced channeling may be due to increased activity of the lysine-sensitive AK isozymes as a result of their lower feedback inhibition by the reduced lysine levels. Alternatively, the DHPS knockout mutation may have triggered enhanced expression of the AK genes and perhaps other genes of the threonine branch of the aspartate family pathway.