It is unique that the structure-function relationships of pyridoxal phosphate-dependent serine (threonine) dehydratase are extremely different from species to species. There exist three types of serine dehydratases; a monomeric type specific for D-serine in some bacteria; a dimeric type found in mammalian livers; and a tetrameric type present in bacteria, yeast, fungi and plants. The tetrameric type consists of biosynthetic and biodegradative serine dehydratases that are involved in the biosynthesis of isoleucine and in growth in medium containing serine or threonine as a sole nitrogen source, respectively. The overall identity of the amino acid sequences of all the enzymes is very low but at least 2 regions are conserved; a segment around the pyridoxal phosphate-binding lysyl residue and a glycine-rich sequence coordinated with the phosphate moiety of the coenzyme. The crystal structure of the Escherichia coli biosynthetic serine dehydratase reveals two domains, an N-terminal catalytic domain and a C-terminal regulatory domain. The subunit of serine dehydratase of rat liver is also suggested to be composed of two domains by limited proteolysis.
Comparative studies reveal that there is a rough inverse correlation between enzyme activity and body size of mammals with a difference of 100-fold magnitude, in which the enzyme is most abundant in rodents and very low in other animals. Serine dehydratase has been demonstrated to contribute to serine metabolism in rat liver. Moreover, a ubiquitous and pivotal role for serine dehydratase in the other animals is not presumed. Rat liver serine dehydratase is induced under various conditions. The gene is organized with 10 exons. Two kinds of mRNA are produced by alternative splicing, of which only one is translated into the enzyme. The gene expression follows a diurnal variation with a peak at the onset of darkness and a nadir at the onset of light, but this rhythmicity is not reflected in the enzyme activity. Recent results with rat liver serine dehydratase are in particular highlighted in the second half of this article.
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