Plant & zo
The science of plants and more
Regulation can happen in many ways. For metabolic pathways three main regulatory mechanisms can be distinguished. A) The production of its enzymes, B) the degradation of these enzymes and C) regulating the activity of these enzymes. Where the regulation through the production and degradation of enzymes is crude and imprecise, regulation through regulating enzyme activity allows for precise reactions of developmental and environmental cues. The recent study of Yokoyama and colleagues on the regulation of the shikimate pathway illustrates this nicely.
Yokoyama and colleagues studied the Arabidopsis DHS enzymes. DHS is the enzyme that catalyses the first reaction of the shikimate pathway, whose products are the aromatic amino acids tyrosine, tryptophan, and phenylalanine. These amino acids are not only needed for protein synthesis but are also functioning as precursors for various secondary metabolites, including auxin, lignin, and flavonoids.
Arabidopsis has three DHS enzymes, DHS1, DHS2 and DHS3. Al three have roughly the same activity, with DHS1 doing slightly better that DHS2 and 3. The main difference between these enzymes is when they are expressed. DHS2 is mainly expressed in seedlings, whereas DHS1 and DHS3 did not show any expression in seedlings but are expressed in mature leaves. Moreover, their expression is also induced in response to stress.
Yokoyama and colleagues also looked at how the DHS enzymes are regulated. They did this trough looking which downstream products inhibited their activity. They found that surprisingly the amino acid phenylalanine did not inhibit any of the DHS enzymes and that the amino acids tryptophan and tyrosine only inhibited DHS2. As DHS2 is the only DHS enzyme active in seedlings, DHS2 inhibition by tryptophan and tyrosine but not phenylalanine likely reflects the needs of the growing seedling, needing amino acids for proteins and lignin, a downstream product of phenylalanine, for its cell walls.
That the activity of DHS1 and 3 are not inhibited by any of the aromatic acids might be correlated with them showing increased expression during stress. The downstream products of all three aromatic amino acids are contributing to the plant stress response, as such it would be damaging to the plant if the production of the aromatic amino acids is limited by their own abundance.
Yokoyama and colleagues did find however, that the activity of all three DHS enzymes is inhibited by chorismate, an intermediate of the shikimate pathway. But that this inhibition has a kind of fail safe for DHS1 and 3. For these enzymes, chorismate inhibition is inhibited by arogenate, the direct precursor of tyrosine and phenylalanine. Kind of telling the enzymes, don’t worry, the chorismate build up is not due to a reduction of flow through the shikimate pathway.
For DHS2 however, arogenate also inhibits its activity. This was also found to be the case for caffeate, a downstream product of phenylalanine and a precursor of lignin. Again, this can be seen as reflecting the need of a growing seedling that does not want to waste its carbon on metabolites that are not used.
The limited photosynthetic activity of seedlings requires them to be thrifty. In contrast mature leaves with their more abundant photosynthetic activity and carbon supply can afford waste carbon on secondary metabolites that come in handy for a range of processes ranging from stress response to pollinator attraction. The DHS enzyme production and inhibition reflects this, fine tuning the shikimate pathway, allowing it to respond to both developmental and environmental cues.
Ryo Yokoyama, Marcos V V de Oliveira, Bailey Kleven, Hiroshi A Maeda (2021) The entry reaction of the plant shikimate pathway is subjected to highly complex metabolite-mediated regulation, The Plant Cell, koaa042