In humans, α-L-Iduronidase (IDUA) is a 653 amino acid protein. IDUA acts on glycosaminoglycans (GAGs) dermatan sulfate and heparan sulfate. Specifically, it breaks a bond between L-iduronic acid (IdoA) and either and either N-acetylgalactosamine-4-sulfate (in dermatan sulfate) or N-sulfo-D-glucosamine-6-sulfate (in heparan sulfate)
Historically, developing a catalytic mechanism for α-L-Iduronidase(IDUA) had been difficult because of difficulties expressing the human protein in mammalian cells. Then, in 2013, the first structures were published using Chinese hamster ovary cells, which consisted of amino acids 27-642 in both the IdoA-bound and apo forms (followed months later by another similar structure paper). IDUA consists of three domains: a TIM barrel, a β-sandwich, and an Ig-like domain. Notably, the authors found that there is extensive N-glycosylation on the residue Asn372. A chain of 2 N-acetylglucosamines and 8 mannose sugars (GlcNAc2Man8) is found on the surface of the TIM barrel, where it is stabilized by water and polar regions of the amino acids. The 7th mannose forms two essential hydrogen bonds with the substrate, and the architecture of the catalytic site is such that the long glycosaminoglycan polymer besides the catalyzed region would be able to exist around the enzyme. Further studies on the deglycosylated form of the enzyme show that the glycosylation at Asn372 is necessary for proper enzyme functionality. (Maita et al. 2013)
Months after the first structure paper was published, another one made it into Nature. The authors confirmed the necessity of the mannose 7 modification, indicating that it forms polar contacts with an oxygen and 5 carbon atoms of IDoA. They reported an over 90% loss of enzymatic activity when IDUA is without its mannose 7, further confirming its role in catalysis. Using IdoA analogs, the authors were able to propose a double-displacement catalytic mechanism utilizing the residue Glu182 as a general acid/base and Glu299 as a nucleophile. Using these conclusions, the authors then connected the enzyme structure to the Hurler Syndrome disease state. They used the common P533R mutation as a model mutation to determine how the enzyme is rendered dysfunctional. While the mutated residue is 25Å away from the electrophilic Glu299, it is on a loop that connects two strands in the β-sandwich domain, which in turn interacts with a helix of the TIM-barrel. The interactions between these domains ultimately position the N-glycan chain on Asn372, which is necessary for catalysis. The mutation from proline to the bulkier and charged arginine can sufficiently disrupt the structure of the enzyme. The chain reaction of disrupted interactions ultimately disrupts the catalytic site. (Bie et al. 2013)
The physical buildup of unreacted GAGs directly correlates to the diseased phenotype. Dermatan and heparan sulfate have been shown to accumulate in the liver and spleen as well as in the lysosomes of cells with extensive connections between cells, like bones, corneas, and connective tissue. (Wang et al. 2010, Liu et al. 2005). This increase in molecular storage means that lysosomes grow to more than 50x their normal volumes and can take up of 20% of cellular volume in Hurler patients (Resnick et al. 1994). In the central nervous system, increased lysosomal volumes causes a manifold phenotype. The meninges swell due to the increase in volume, which prevents cerebrospinal fluid movement, and pressure on the brain and spinal cord increases. Taken together, these processes are thought to cause the mental retardation and hernia symptoms (Gabrielli et al. 2003).