Hint, Fhit, and GalT:  Function, Structure, Evolution, and Mechanism of Three Branches of the Histidine Triad Superfamily of Nucleotide Hydrolases and Transferases

Abstract

HIT (histidine triad)1 proteins, named for a motif related to the sequence HφHφHφφ, (φ a hydrophobic amino acid) are a superfamily of nucleotide hydrolases and transferases, which act on the α-phosphate of ribonucleotides, and contain a ∼30 kDa domain that is typically either a homodimer of ∼15 kDa polypeptides with two active-sites or an internally, imperfectly repeated polypeptide that retains a single HIT active site. On the basis of sequence, substrate specificity, structure, evolution and mechanism, HIT proteins can be classified into the Hint branch, which consists of adenosine 5′-monophosphoramide hydrolases, the Fhit branch, which consists of diadenosine polyphosphate hydrolases, and the GalT branch, which consists of specific nucleoside monophosphate transferases including galactose-1-phosphate uridylyltransferase, diadenosine tetraphosphate phosphorylase, and adenylylsulfate:phosphate adenylytransferase. At least one human representative of each branch is lost in human diseases. Aprataxin, a Hint branch hydrolase, is mutated in ataxia-oculomotor apraxia syndrome. Fhit is lost early in development of many epithelially derived tumors. GalT is deficient in galactosemia. Additionally, ASW is an avian Hint family member that has evolved to have unusual gene expression properties and the complete loss of its nucleotide binding-site. The potential roles of ASW and Hint in avian sexual development are discussed in an accompanying manuscript. Here we review what is known about biological activities of HIT proteins, the structural and biochemical bases for their functions, and propose a new enzyme mechanism for Hint and Fhit that may account for the differences between HIT hydrolases and transferases. Mammalian Hint and GalT Structurally Define the HIT SuperfamilyGalactose-1-phosphate uridylyltransferase (GalT), the second enzyme in the Leloir pathway of galactose utilization (1), was crystallized and its structure determined to understand the mechanistic basis for transferring UMP between glucose-1-phosphate and galactose-1-phosphate (2-4). Histidine triad nucleotide-binding protein (Hint), purified as a purine nucleoside/nucleotide-binding protein from rabbit heart cytosol (5) and shown to have a widely conserved sequence (6), was crystallized and its structure determined to establish whether the conserved sequence formed the basis for a new mode of nucleotide-binding (7). GalT (2) and Hint are both dimers (8) but the GalT monomer is more than twice the size of the Hint dimer. Though GalT (9) and Hint (6) homologs could be cloned by similarity and the crystal structures (2,8) co-existed in protein databases, mutual similarity was not noted until inspection of the rabbit Hint crystal structure (7) and computational analysis (10) indicated that a GalT monomer and a Hint dimer can be superimposed and that their nucleotide-binding sites retain some of the same residues for binding a nucleoside monophosphate (7) (Figure 1). The level of sequence similarity of Hint and GalT is difficult to distinguish from n...

Gene Expression of Metabolic Enzymes and a Protease Inhibitor in the Prefrontal Cortex Are Decreased in Schizophrenia

“…One possibility is the HINT enzymes have adenylated proteins as substrates, although the potential ligands of HINT and subsequent reactants remain unknown. A loss of HINT activity restricted growth rate in yeast (19,21,23,24). Thus, a 35% reduction in HINT1 enzyme mRNA in neurons in cortical layer VI (21% decrease in layers II-V) might be the result of abnormal cell transcription, although results in yeast leading to alterations in cell growth rate do not translate into alterations in the growth of neurons that are terminally differentiated cells.…”

“…The human gene, HINT1, is a member of the histidine triad superfamily of nucleotide hydrolases. Nucleotide transferases consist of three branches: proteins related to Hint and Aprataxin, a branch of Fhit-related hydrolases, and a branch of galactose-1-phosphate uridylytransferase (GalT)-related transferases (19). A mutation of a gene in these branches has been associated with functional consequences such as ataxia-oculomotor apraxia syndrome, epithelially derived tumors, and galactosemia (19).…”

“…However, no grossly observable deficiencies were noted in hint Ϫ/Ϫ mice, suggesting that other redundant genes can functionally compensate (20). HINT enzyme homologues hydrolyze adenosine-5Ј-monophosphoramidate (AMPNH 2 ) to AMP plus the leaving NH 2 group (19,21). HINT enzyme activity is a positive regulator of Kin28, the human homologue is CDK7 (21).…”

Early steps in the DNA base excision/single-strand interruption repair pathway in mammalian cells

“…Aprataxin belongs to a family of nucleotide hydrolases and transferases [135,136]. Mutations in the aprataxin gene…”

Laboratory diagnosis of galactosemia: a technical standard and guideline of the American College of Medical Genetics and Genomics (ACMG)

“…2 This enzyme catalyzes the interconversion of Gal-1-P to glucose-1-phosphate, with the concomitant interconversion of the co-substrate UDP-glucose to UDPgalactose. The reaction follows a "ping-pong" mechanism whereby histidine at the active site reacts with UDP-glucose loading the enzyme with uridine 5′-monophosphate (UMP); Figure 1 Metabolism of galactose.…”