br Acknowledgments We apologize for the omission of

Acknowledgments We apologize for the omission of primary citations owing to space limitations. This work was supported by grants from National Natural Science Foundation of China (81772801 and 81472455 to C.D.), the Key Program of Zhejiang Provincial Natural Science Foundation (LZ17H160002 to C.D.), the National Key R&D Program of China (2016YFC1303200 to C.D.), the Fundamental Research Funds for Central Universities of China (to C.D.), and the Thousand Young Talents Plan of China (to C.D.).
Introduction In 1999, the term “moonlighting” was cued (Jeffery, 1999). It refers to the phenomenon of metabolic enzymes displaying unrelated functional activities, excluding functions which result from alternative mRNA splicing or post-translational modifications of a protein. Enzymes of carbohydrate metabolism were found to play “additional” roles in regulation of transcription, DNA reperation, cell motility and apoptosis (reviewed e.g. in Kim and Dang, 2005). Fructose-1,6-bisphosphatase (FBPase; EC 3.1.3.11), described for the first time in 1943 (Gomori, 1943) for a long time seemed not to fit in with this term. Catalysing the hydrolysis of fructose-1,6-bisphosphate (F-1,6-BP) to fructose-6-phosphate (F-6-P) and inorganic phosphate FBPase appeared to be just a regulatory enzyme of gluconeogenesis and glyconeogenesis (Tejwani, 1983), i.e. glucose and glycogen synthesis from carbohydrate precursors. As such, FBPase was not extensively studied (except at the structural level). Over the years, accumulating pieces of evidence challenged this simplified picture of FBPase, unveiling different, more intricate truth about its functional activity in a cell. Invertebrate genomes contain a single fbp locus (Tillmann et al., 2002). In vertebrates, there are two distinct genes, fbp1 and fbp2, encoding two FBPase isozymes. Liver FBPase (FBP1), the protein product of the fbp1 gene, is expressed mainly in gluconeogenic organs (Al-Robaiy and Eschrich, 1999) while muscle FBPase (FBP2), encoded by fbp2, is widely expressed in vertebrate cells, not only in glyconeogenic ones (e.g. striated muscle fibres) but also in Sephin1 such as neurons which are not thought to synthesize glycogen from carbohydrate precursors (Loffler et al., 2001). FBP2 is also expressed in cells predominantly producing the liver isozyme, e.g. in the liver itself (Al-Robaiy and Eschrich, 1999). The discovery of the ubiquitous expression of FBP2 raised a question about the role of the isozyme in cell biology and re-awakened interest in the protein. Henceforward, numerous studies provided pieces of evidence of unexpected nature of FBPase.
Structural and kinetic properties of FBPase – the origin and physiological meaning of distinct sensitivity of two isozymes towards effectors Most of the kinetic and structural studies have been performed using FBP1 (isolated from mammalian liver and kidney), however, because of a high similarity of the catalytic and substrate-binding sites between the isozymes (Al-Robaiy and Eschrich, 1999), entirely the same mechanism of catalysis is expected to occur in the muscle and liver FBPase. Biochemical, genetic and crystallographic studies have revealed that mammalian FBPases are homotetrameric enzymes with a subunit molecular mass of about 37 kDa (Marcus et al., 1982; El-Maghrabi et al., 1993; Tillmann and Eschrich, 1998; Al-Robaiy and Eschrich, 1999; Skalecki et al., 1999). Tertiary structure of each monomer is composed of two domains, a domain containing the substrate binding site (FBP domain) and an allosteric domain which may interact with AMP (AMP domain) (Ke et al., 1990; Villeret et al., 1995; Choe et al., 2000) (Fig. 1). Within each monomer the allosteric site is approximately 28 Å away from the active site which is located between FBP and AMP domain (Ke et al., 1990; Villeret et al., 1995; Choe et al., 2000). The mechanism of the catalysis requires binding of divalent cations such as Mg2+, Mn2+, Co2+ or Zn2+ (Benkovic and deMaine, 1982; Tejwani, 1983), which all probably bind to the 1-phosphoryl group of fructose-1,6-bisphosphate (Scheffler and Fromm, 1996; Choe et al., 2000). Both isozymes are activated by some monovalent cations, such as Na+, K+ and NH4+ (Benkovic and deMaine, 1982; Tejwani, 1983; Zhang et al., 1996) (Fig. 1).