Acetyl-coenzyme A:alpha-glucosaminide N-acetyltransferase. Evidence for an active site histidine residue

Structure and mechanism of lysosome transmembrane acetylation by HGSNAT

Lysosomal transmembrane acetylation of heparan sulfates (HS) is catalyzed by HS acetyl-CoA:α-glucosaminide N-acetyltransferase (HGSNAT), whose dysfunction leads to lysosomal storage diseases. The mechanism by which HGSNAT, the sole non-hydrolase enzyme in HS degradation, brings cytosolic acetyl-coenzyme A (Ac-CoA) and lysosomal HS together for N-acyltransferase reactions remains unclear. Here, we present cryogenic-electron microscopy structures of HGSNAT alone, complexed with Ac-CoA and with acetylated products. These structures explain that Ac-CoA binding from the cytosolic side causes dimeric HGSNAT to form a transmembrane tunnel. Within this tunnel, catalytic histidine and asparagine approach the lumen and instigate the transfer of the acetyl group from Ac-CoA to the glucosamine group of HS. Our study unveils a transmembrane acetylation mechanism that may help advance therapeutic strategies targeting lysosomal storage diseases.

Structure of the human heparan-α-glucosaminide N-acetyltransferase (HGSNAT)

Degradation of heparan sulfate (HS), a glycosaminoglycan (GAG) comprised of repeating units of N-acetylglucosamine and glucuronic acid, begins in the cytosol and is completed in the lysosomes. Acetylation of the terminal non-reducing amino group of α-D-glucosamine of HS is essential for its complete breakdown into monosaccharides and free sulfate. Heparan-α-glucosaminide N-acetyltransferase (HGSNAT), a resident of the lysosomal membrane, catalyzes this essential acetylation reaction by accepting and transferring the acetyl group from cytosolic acetyl-CoA to terminal α-D-glucosamine of HS in the lysosomal lumen. Mutation-induced dysfunction in HGSNAT causes abnormal accumulation of HS within the lysosomes and leads to an autosomal recessive neurodegenerative lysosomal storage disorder called mucopolysaccharidosis IIIC (MPS IIIC). There are no approved drugs or treatment strategies to cure or manage the symptoms of, MPS IIIC. Here, we use cryo-electron microscopy (cryo-EM) to determine a high-resolution structure of the HGSNAT-acetyl-CoA complex, the first step in the HGSNAT-catalyzed acetyltransferase reaction. In addition, we map the known MPS IIIC mutations onto the structure and elucidate the molecular basis for mutation-induced HGSNAT dysfunction.

Structural and mechanistic insights into a lysosomal membrane enzyme HGSNAT involved in Sanfilippo syndrome

Heparan sulfate (HS) is degraded in lysosome by a series of glycosidases. Before the glycosidases can act, the terminal glucosamine of HS must be acetylated by the integral lysosomal membrane enzyme heparan-α-glucosaminide N-acetyltransferase (HGSNAT). Mutations of HGSNAT cause HS accumulation and consequently mucopolysaccharidosis IIIC, a devastating lysosomal storage disease characterized by progressive neurological deterioration and early death where no treatment is available. HGSNAT catalyzes a unique transmembrane acetylation reaction where the acetyl group of cytosolic acetyl-CoA is transported across the lysosomal membrane and attached to HS in one reaction. However, the reaction mechanism remains elusive. Here we report six cryo-EM structures of HGSNAT along the reaction pathway. These structures reveal a dimer arrangement and a unique structural fold, which enables the elucidation of the reaction mechanism. We find that a central pore within each monomer traverses the membrane and controls access of cytosolic acetyl-CoA to the active site at its luminal mouth where glucosamine binds. A histidine-aspartic acid catalytic dyad catalyzes the transfer reaction via a ternary complex mechanism. Furthermore, the structures allow the mapping of disease-causing variants and reveal their potential impact on the function, thus creating a framework to guide structure-based drug discovery efforts.

Structure of the human heparan-α-glucosaminide N-acetyltransferase (HGSNAT)

Degradation of heparan sulfate (HS), a glycosaminoglycan (GAG) comprised of repeating units of N-acetylglucosamine and glucuronic acid, begins in the cytosol and is completed in the lysosomes. Acetylation of the terminal non-reducing amino group of a-D-glucosamine of HS is essential for its complete breakdown into monosaccharides and free sulfate. Heparan-a-glucosaminide N-acetyltransferase (HGSNAT), a resident of the lysosomal membrane, catalyzes this essential acetylation reaction by accepting and transferring the acetyl group from cytosolic acetyl-CoA to terminal a-D-glucosamine of HS in the lysosomal lumen. Mutation-induced dysfunction in HGSNAT causes abnormal accumulation of HS within the lysosomes and leads to an autosomal recessive neurodegenerative lysosomal storage disorder called mucopolysaccharidosis IIIC (MPS IIIC). There are no approved drugs or treatment strategies to cure or manage the symptoms of, MPS IIIC. Here, we use cryo-electron microscopy (cryo-EM) to determine a high-resolution structure of the HGSNAT-acetyl-CoA complex, the first step in HGSNAT catalyzed acetyltransferase reaction. In addition, we map the known MPS IIIC mutations onto the structure and elucidate the molecular basis for mutation-induced HGSNAT dysfunction.

Mechanism of Staphylococcus aureus peptidoglycan O-acetyltransferase A as an O-acyltransferase

The O-acetylation of exopolysaccharides, including the essential bacterial cell wall polymer peptidoglycan, confers resistance to their lysis by exogenous hydrolases. Like the enzymes catalyzing the O-acetylation of exopolysaccharides in the Golgi of animals and fungi, peptidoglycan O-acetyltransferase A (OatA) is predicted to be an integral membrane protein comprised of a membrane-spanning acyltransferase-3 (AT-3) domain and an extracytoplasmic domain; for OatA, these domains are located in the N- and C-terminal regions of the enzyme, respectively. The recombinant C-terminal domain (OatA_C) has been characterized as an SGNH acetyltransferase, but nothing was known about the function of the N-terminal AT-3 domain (OatA_N) or its homologs associated with other acyltransferases. We report herein the experimental determination of the topology of Staphylococcus aureus OatA_N, which differs markedly from that predicted in silico. We present the biochemical characterization of OatA_N as part of recombinant OatA and demonstrate that acetyl-CoA serves as the substrate for OatA_N Using in situ and in vitro assays, we characterized 35 engineered OatA variants which identified a catalytic triad of Tyr-His-Glu residues. We trapped an acetyl group from acetyl-CoA on the catalytic Tyr residue that is located on an extracytoplasmic loop of OatA_N Further enzymatic characterization revealed that O-acetyl-Tyr represents the substrate for OatA_C We propose a model for OatA action involving the translocation of acetyl groups from acetyl-CoA across the cytoplasmic membrane by OatA_N and their subsequent intramolecular transfer to OatA_C for the O-acetylation of peptidoglycan via the concerted action of catalytic Tyr and Ser residues.

HGSNAT has a TATA-less promoter with multiple starts of transcription

Acetyl-CoA:α-glucosaminide N-acetyltransferase (N-acetyltransferase) is a lysosomal membrane enzyme that catalyzes a key step in the lysosomal degradation of heparan sulfate. Its deficiency causes Sanfilippo syndrome type IIIC (Mucopolysaccharidosis type IIIC, MPS IIIC). Here we characterize the promoter region of HGSNAT, the gene encoding N-acetyltransferase, which is located in the pericentromeric region of chromosome 8. We show that HGSNAT transcription is driven by a TATA-less promoter whose key elements are contained within the 1054bp region upstream of exon 1. About 400 bases of the region's 3'-prime end overlap with an unmethylated CpG island. Reduced reporter activities from promoter serial deletion constructs suggested strong regulatory elements at positions -101 to -20bp and -1073 to -716bp of the downstream initiation codon (DS-ATG). Targeted mutagenesis of the first Specificity protein 1-A (Sp1-A) of the six in silico-predicted Sp1 sites in the region flanking the major transcription start sites (TSSs, +50/-101) led to a 55% decrease of reporter activity, while inactivation of each of Sp1-B and Sp1-C resulted in its almost two-fold increase. The binding of Sp1 to the region was confirmed by chromatin immunoprecipitation (ChIP). Overall, this confirms that Sp1 is important for regulation of the HGSNAT promoter. Promoter fragments in antisense orientation (constructs pGL4 -20/-1305 and pGL4 +50/-1305) led to reporter activities of about 50% of the pGL4 -1305/-20 activity, implying divergent initiation of transcription at the promoter. We identified two main TSSs at positions +1 and -15 from DS-ATG using Rapid amplification of cDNA ends (5'RACE). Transcripts initiating at the TSSs thus contain only DS-ATG. Five patients from our MPS IIIC cohort (n=23) carried the rs4523300 promoter variant and one the rs149596192 promoter variant. Both variants lowered the expression of the reporter down to 68% and 59%, respectively. However, white blood cell (WBC) N-acetyltransferase activities in individuals carrying the variants did not significantly differ from homozygotes for the wild-type alleles, suggesting only a partial impact of transcriptional regulation on N-acetyltransferase activities in vivo.

Analysis of the biogenesis of heparan sulfate acetyl-CoA:alpha-glucosaminide N-acetyltransferase provides insights into the mechanism underlying its complete deficiency in mucopolysaccharidosis IIIC

Heparan sulfate acetyl-CoA:α-glucosaminide N-acetyltransferase (HGSNAT) catalyzes the transmembrane acetylation of heparan sulfate in lysosomes required for its further catabolism. Inherited deficiency of HGSNAT in humans results in lysosomal storage of heparan sulfate and causes the severe neurodegenerative disease, mucopolysaccharidosis IIIC (MPS IIIC). Previously we have cloned the HGSNAT gene, identified molecular defects in MPS IIIC patients, and found that all missense mutations prevented normal folding and trafficking of the enzyme. Therefore characterization of HGSNAT biogenesis and intracellular trafficking became of central importance for understanding the molecular mechanism underlying the disease and developing future therapies. In the current study we show that HGSNAT is synthesized as a catalytically inactive 77-kDa precursor that is transported to the lysosomes via an adaptor protein-mediated pathway that involves conserved tyrosine- and dileucine-based lysosomal targeting signals in its C-terminal cytoplasmic domain with a contribution from a dileucine-based signal in the N-terminal cytoplasmic loop. In the lysosome, the precursor is cleaved into a 29-kDa N-terminal α-chain and a 48-kDa C-terminal β-chain, and assembled into active ∼440-kDa oligomers. The subunits are held together by disulfide bonds between at least two cysteine residues (Cys(123) and Cys(434)) in the lysosomal luminal loops of the enzyme. We speculate that proteolytic cleavage allows the nucleophile residue, His(269), in the active site to access the substrate acetyl-CoA in the cytoplasm, for further transfer of the acetyl group to the terminal glucosamine on heparan sulfate. Altogether our results identify intralysosomal oligomerization and proteolytic cleavage as two steps crucial for functional activation of HGSNAT.

Sanfilippo syndrome: a mini-review

Mucopolysaccharidosis type III (MPS III, Sanfilippo syndrome) is an autosomal recessive disorder, caused by a deficiency in one of the four enzymes involved in the lysosomal degradation of the glycosaminoglycan heparan sulfate. Based on the enzyme deficiency, four different subtypes, MPS IIIA, B, C, and D, are recognized. The genes encoding these four enzymes have been characterized and various mutations have been reported. The probable diagnosis of all MPS III subtypes is based on increased concentration of heparan sulfate in the urine. Enzymatic assays in leukocytes and/or fibroblasts confirm the diagnosis and allow for discrimination between the different subtypes of the disease. The clinical course of MPS III can be divided into three phases. In the first phase, which usually starts between 1 and 4 years of age, a developmental delay becomes apparent after an initial normal development during the first 1-2 years of life. The second phase generally starts around 3-4 years and is characterized by severe behavioural problems and progressive mental deterioration ultimately leading to severe dementia. In the third and final stage, behavioural problems slowly disappear, but motor retardation with swallowing difficulties and spasticity emerge. Patients usually die at the end of the second or beginning of the third decade of life, although survival into the fourth decade has been reported. Although currently no effective therapy is yet available for MPS III, several promising developments raise hope that therapeutic interventions, halting the devastating mental and behavioural deterioration, might be feasible in the near future.

Mutations in TMEM76* cause mucopolysaccharidosis IIIC (Sanfilippo C syndrome)

Mucopolysaccharidosis IIIC (MPS IIIC, or Sanfilippo C syndrome) is a lysosomal storage disorder caused by the inherited deficiency of the lysosomal membrane enzyme acetyl-coenzyme A: alpha -glucosaminide N-acetyltransferase (N-acetyltransferase), which leads to impaired degradation of heparan sulfate. We report the narrowing of the candidate region to a 2.6-cM interval between D8S1051 and D8S1831 and the identification of the transmembrane protein 76 gene (TMEM76), which encodes a 73-kDa protein with predicted multiple transmembrane domains and glycosylation sites, as the gene that causes MPS IIIC when it is mutated. Four nonsense mutations, 3 frameshift mutations due to deletions or a duplication, 6 splice-site mutations, and 14 missense mutations were identified among 30 probands with MPS IIIC. Functional expression of human TMEM76 and the mouse ortholog demonstrates that it is the gene that encodes the lysosomal N-acetyltransferase and suggests that this enzyme belongs to a new structural class of proteins that transport the activated acetyl residues across the cell membrane.

An acetylated 120-kDa lysosomal transmembrane protein is absent from mucopolysaccharidosis IIIC fibroblasts: a candidate molecule for MPS IIIC

Genetic deficiency of the lysosomal enzyme, acetyl-CoA: alpha-glucosaminide N-acetyltransferase (N-acetyltransferase), which catalyses the transmembrane acetylation of heparan sulfate results in severe neurodegenerative disease, mucopolysaccharidosis IIIC. N-Acetyltransferase has never been characterized structurally and its gene has never been identified. We combined traditional methods of enzyme purification with organellar proteomics, isolating lysosomal membranes from mouse liver using differential centrifugation and osmolysis, followed by detergent extraction and purification of N-acetyltransferase by liquid chromatography. Partially purified enzyme had a molecular mass of 240 kDa and pI of 7.4 by gel filtration and chromatofocusing. Its specific activity varied with protein concentration typical of oligomeric enzymes or multienzyme complexes. Incubation of N-acetyltransferase with acetyl[14C]CoA in the absence of the acceptor of the acetyl group resulted in radioactive labeling of a 120-kDa polypeptide, suggesting that it represents a subunit containing the enzyme active site. Furthermore, following acetyl[14C]-labeling, the 120-kDa protein was present in the lysosomal membranes purified from the normal human skin fibroblasts but absent in those from the skin fibroblasts of MPS IIIC patients.

Identification of specific histidine residues and the carboxyl terminus are essential for serotonin N-acetyltransferase enzymatic activity

Melatonin is synthesized in pinealocytes of the pineal gland and in photoreceptors of the retina. Synthesis rate from serotonin to melatonin is controlled by the rapid and dramatic enzymatic increase in darkness of serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AA-NAT, EC 2.3.1.87) and hydroxyindole-O-methyltransferase (HIOMT, EC 2.1.1.4). The primary structure of these critical indoleamine enzymes is now known and the regulation of the enzyme catalysis can be examined. As a first step, the conserved cysteine (C) and histidine (H) residues were targeted for site-directed mutagenesis as potential amino acid residues involved in the N-acetylation reaction of AA-NAT. Our studies concluded that among 6 histidine (H) to alanine (A) mutations, three residues (H110A, H118A, H120A) within the AA-NAT protein showed little or no enzymatic activity, whereas the others (H28A, H70A, H125A) retained enzymatic activity, compared to the unaltered AA-NAT protein. Cysteine to alanine mutations, C37A and C177A, had no significant effect on the AA-NAT enzymatic activity; however, C61A had a four-fold increase in K(m) for acetyl CoA and an altered sensitivity to the thiol modification chemical, N-ethylmaleimide (NEM), implying that C61 may participate in the acetyl CoA binding. Further studies examined the AA-NAT enzyme regulation of the highly conserved carboxyl terminus. When 12 terminal amino acid residues were deleted systematically from the carboxyl terminus of the 205 amino acid residue AA-NAT protein, enzyme activity was retained. However, further residue deletion resulted in enzyme activity plummeting, implicating that the essential information either for the correct structural folding into an active enzyme form or for enzyme stability is in the 193 residues. To test the relative importance of the AA-NAT carboxyl terminal region, a single leucine (L) was altered to alanine (A) or proline (P). Both mutants, either L193A or L193P, had a marked decrease in AA-NAT enzymatic activity and a decrease in thermal stability, suggesting the leucine, in addition to the cysteine and histidine residues, is involved in either enzyme catalysis or stability. In light of the recently reported three-dimensional structure of AA-NAT (17,18), the site-directed mutagenesis data demonstrate experimentally the importance of essential amino acid residues for acetyl CoA binding and AA-NAT activation.

Biochemical characterization of recombinant serotonin N-acetyltransferase

Pineal and retinal melatonin synthesis is controlled by the enzymatic activity of arylalkylamine N-acetyltransferase (AA-NAT, EC 2.3.1.87), which is regulated by light/dark signals and circadian factors. This enzyme converts serotonin to N-acetylserotonin by the transfer of an acetyl group from acetyl coenzyme A. Endogenous AA-NAT instability during routine purification has made enzyme characterization difficult, but now a stable recombinant protein for AA-NAT has been synthesized to investigate the intrinsic biochemical properties of AA-NAT from a rat pineal cDNA encoding a 205 amino acid, 23 kilodalton protein, by using a glutathione-S-transferase (GST) fusion protein system. Recombinant GST-AA-NAT showed substrate specificity for arylalkylamines and stability at 4 degrees C; however, the enzyme activity was reduced by 40% upon preincubation at 37 degrees C for 2 hr. GST-AA-NAT is preferentially phosphorylated by either cyclic AMP- or cyclic GMP-dependent kinases in vitro, but no detrimental effect was observed on AA-NAT enzymatic activity. Among the metal cations tested in this study, Ca2+, Mg2+, Mn2+, Fe2+, and Co2 showed little or no inhibitory potency, while either 1 mM Zn2+ or 0.1 mM Cu2+ nearly abolished the enzymatic activity. GST-AA-NAT enzyme activity is also inhibited by reagents that are known biochemically to modify thiol groups (N-ethylmaleimide, NEM) and histidine residues (p-chloromercuribenzoate, NBS and diethyl pyrocarbonate, DEPC), suggesting the presence of essential cysteine and histidine moieties. Moreover, preincubation of acetyl CoA completely protects the recombinant AA-NAT from inactivation by NEM and DEPC, indicating that specific cysteine and histidine residues may be at the acetylation site. The conclusion is that the biochemical properties of rat recombinant AA-NAT is similar to the endogenous pineal and retinal AA-NAT with respect to the sensitivity to temperature, metal cations, as well as the thiol modification reagents. These data also suggest that the phosphorylation status of the AA-NAT does not affect enzymatic activity directly, and histidine residues are potentially important residues required for high catalytic activity.

Genetic evidence for transmembrane acetylation by lysosomes

Acetyl-CoA:alpha-glucosaminide N-acetyltransferase is a lysosomal-membrane enzyme deficient in a genetic disorder, Sanfilippo disease type C. The enzyme catalyzes the transfer of an acetyl group from cytoplasmic acetyl-coenzyme A (acetyl-CoA) to terminal alpha-glucosamine residues of heparan sulfate within the organelle. Previous kinetic experiments indicated that the enzyme carries out a transmembrane acetylation via a ping-pong mechanism; the reaction can therefore be dissected into two half reactions--acetylation of the enzyme, and transfer of the acetyl group to glucosamine. Cells derived from patients were found to differ in their ability to perform each half reaction. Five cell lines (derived from three families) were able to catalyze acetylation of the lysosomal membrane and to carry out acetyl-CoA/CoA exchange, whereas a sixth cell line was devoid of this activity.