Iduronidase protein structure and processing. The iduronidase protein is initially synthesized as a 653-amino acid polypeptide and the 25-amino acid signal sequence is clipped off during translation. The protein has six N-linked oligosaccharides added during translation. After carbohydrate addition, the sugars undergo trimming and additions leading to complex or hybrid chains (C), high-mannose 5-9 chains (M), and phosphorylated high-mannose chains (PO4).
with histidine, a polymorphism present in about 9% of the Caucasian population . There is no known effect of this polymorphism on enzyme activity or stability, although other polymorphisms may affect residual activity of mutant iduronidase proteins, which may account for some heterogeneity between patients with similar mutations.
After synthesis in the rough endoplasmic reticulum, the signal peptide is cleaved. Although the processing step is not well defined for normal a-L-iduronidase due to the difficulties in analyzing the very low normal levels of endogenous protein, the recombinant protein has A26 at its N terminus (Figure 12.2). This N terminus is not well predicted by the von Heijne rules for signal peptide cleavage and so it is not clear if further trimming occurs after the initial signal peptide cleavage . After transport to the lysosome, further clipping of the N terminus can occur and so the terminal amino acids from purified native human a-L-iduronidase may not reflect the original N-terminal cleavage site [7,9]. After transport to the lysosome, recombinant a-L-iduronidase also undergoes an additional N-terminal cleavage that is documented by pulse-chase studies, but it is not clear whether this clip leads to the dissociation of the clipped peptide from the protein, or whether this N-terminal piece remains associated with the enzyme .
The complete tertiary structure for a-L-iduronidase has not been determined by X-ray crystallography, although there are structure models proposed using predictive rules based on primary enzyme sequence . The tertiary structure of the protein is active as a monomer in solution in normal salt conditions. No consistent higher-order structure has been described or appears to be required for enzyme activity, unlike P-glucuronidase, for example, which exists normally as a homotetramer and needs to be a homodimer to attain enzyme activity.
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