Multifunctional Fructose 1,6-Bisphosphate Aldolase as a Therapeutic Target

Abstract

Fructose 1,6-bisphosphate aldolase is a ubiquitous cytosolic enzyme that catalyzes the fourth step of glycolysis. Aldolases are classified into three groups: Class-I, Class-IA, and Class-II; all classes share similar structural features but low amino acid identity. Apart from their conserved role in carbohydrate metabolism, aldolases have been reported to perform numerous non-enzymatic functions. Here we review the myriad "moonlighting" functions of this classical enzyme, many of which are centered on its ability to bind to an array of partner proteins that impact cellular scaffolding, signaling, transcription, and motility. In addition to the cytosolic location, aldolase has been found the extracellular surface of several pathogenic bacteria, fungi, protozoans, and metazoans. In the extracellular space, the enzyme has been reported to perform virulence-enhancing moonlighting functions e.g., plasminogen binding, host cell adhesion, and immunomodulation. Aldolase's importance has made it both a drug target and vaccine candidate. In this review, we note the several inhibitors that have been synthesized with high specificity for the aldolases of pathogens and cancer cells and have been shown to inhibit classical enzyme activity and moonlighting functions. We also review the many trials in which recombinant aldolases have been used as vaccine targets against a wide variety of pathogenic organisms including bacteria, fungi, and metazoan parasites. Most of such trials generated significant protection from challenge infection, correlated with antigen-specific cellular and humoral immune responses. We argue that refinement of aldolase antigen preparations and expansion of immunization trials should be encouraged to promote the advancement of promising, protective aldolase vaccines.

Keywords: aldolase; glycolysis; inhibitor; moonlighting function; vaccine.

FIGURE 1

The fourth step in glycolysis: fructose 1,6-bisphosphate aldolase (FBA) catalyzes the reversible conversion of the six-carbon glycolytic enzyme fructose 1,6-bisphosphate (F1,6BP, left) into two three-carbon intermediates glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP, right).

FIGURE 2

Phylogenetic tree of fructose 1,6-bisphosphate aldolases constructed by Molecular Evolutionary Genetics Analysis (MEGA) Version 7.0.21 software using amino acid sequences obtained from the UniProt database. The sequences were aligned by BioEdit Sequence Alignment Editor software using ClustalW Multiple sequence alignment. The evolutionary history was inferred using the Neighbor-Joining method. The optimal tree with the sum of branch length = 10.38 is shown. The percentage of replicate trees in which the associated taxa clustered together in the bootstrap test (1,000 replicates) are shown next to the branches. The tree is drawn to scale, with branch lengths in the same units as those of the evolutionary distances used to infer the phylogenetic tree. Evolutionary distances were computed using the Poisson correction method and are in the units of the number of amino acid substitutions per site. The analysis involved 24 amino acid sequences. The UniProt accession numbers for the sequences are as follows: Candida albicans (Q9URB4); Caenorhabditis elegans FBA 1 (P54216); Caenorhabditis elegans (P46563); Drosophila melanogaster (P07764); Escherichia coli Class-I (P0A991); Escherichia coli Class-II (P0AB71); Francisella novicida (A0A2L1CHL1); G. lamblia (O97447); Haloferax volcanii (D4GYE0); Homo sapiens ALDOA (P04075); Homo sapiens ALDOB (P05062); Homo sapiens ALDOC (P09972); Mus musculus ALDOA (P05064); Mus musculusn ALDOB (Q91Y97); Mus musculus ALDOC (P05063); Nicotiana tabacum (F2VJ75); Phaeodactylum tricornutum Class-I (B7GE67); Phaeodactylum tricornutum Class-II (B7G4R3); Plasmodium falciparum (P14223); Pyrococcus furiosus (P58314); Saccharomyces cerevisiae (P14540); Schistosoma mansoni (P53442); Thermoproteus tenax (P58315); Trypanosoma brucei (P07752).

FIGURE 3

A list of bacterial, fungal, protozoan, and metazoan pathogens reported to possess surface-localized fructose 1,6-bisphosphate aldolase (FBA) (left box) and a list of pathogens whose FBA has been reported to perform a specific moonlighting function: that is of binding plasminogen (top right box) and subsequently promoting its conversion into plasmin (top right box, insert). The conversion of plasminogen into plasmin (illustrated in the bottom right box) is catalyzed by protease activators including tissue plasminogen activator (tPA) or urokinase-type plasminogen activator (uPA). The outcome is fibrinolysis (since plasmin cleaves fibrin blood clots) and/or extracellular matrix (ECM) damage (since plasmin cleaves ECM proteins such as fibronectin and laminin). These mechanisms can be exploited by pathogens to favor their invasion and survival within their hosts in vivo, as FBA has been reported to enhance (green arrow) plasmin activation in the presence of activators. Abbreviations: FBA, fructose 1,6-bisphosphate aldolase; ECM, extracellular matrix; tPA, tissue plasminogen activator; uPA, urokinase-type plasminogen activator.