ABSTRACT
BACKGROUND: The crystal structure of precorrin-8x methyl mutase (CobH), an enzyme of the aerobic pathway to vitamin B12, provides evidence that the mechanism for methyl migration can plausibly be regarded as an allowed [1,5]-sigmatropic shift of a methyl group from C-11 to C-12 at the C ring of precorrin-8x to afford hydrogenobyrinic acid. RESULTS: The dimeric structure of CobH creates a set of shared active sites that readily discriminate between different tautomers of precorrin-8x and select a discrete tautomer for sigmatropic rearrangement. The active site contains a strictly conserved histidine residue close to the site of methyl migration in ring C of the substrate. CONCLUSION: Analysis of the structure with bound product suggests that the [1,5]-sigmatropic shift proceeds by protonation of the ring C nitrogen, leading to subsequent methyl migration.
Subject(s)
Bacterial Proteins , Intramolecular Transferases/chemistry , Intramolecular Transferases/metabolism , Amino Acid Sequence , Base Sequence , Binding Sites , Crystallography, X-Ray , Models, Molecular , Molecular Sequence Data , Protein Conformation , Sequence Homology, Amino Acid , Substrate Specificity , Uroporphyrins/chemistry , Uroporphyrins/metabolismABSTRACT
Congenital hypothyroid dwarfism was diagnosed in a family of Giant Schnauzers. Three female and two male puppies from different litters were evaluated for dwarfism, lethargy, somnolence, gait abnormalities, and constipation. On physical examination, disproportionate dwarfism (n = 5), macroglossia (n = 3), hypothermia (n = 3), delayed dental eruption (n = 3), ataxia (n = 2), and abdominal distension (n = 1) were identified. Results of initial laboratory tests showed anemia (n = 4), hypercholesterolemia (n = 4), hypercalcemia (n = 2), and transudative abdominal effusion (n = 1). Radiographic skeletal surveys disclosed epiphyseal dysgenesis and delayed skeletal maturation (n = 5). A diagnosis of hypothyroidism was established on the basis of low basal serum thyroxine concentrations that failed to increase following the administration of TSH (n = 5) and markedly reduced to absent thyroid image when evaluated with gamma camera imaging of the thyroid gland (n = 4). In the two dogs that were most thoroughly evaluated, the results of thyroid histology, prolonged TSH testing, and repeat thyroid imaging, after three daily injections of TSH, were all consistent with secondary or tertiary, rather than primary, hypothyroidism. When TSH was administered over a period of 3 consecutive days (5 IU/day, subcutaneously), serum thyroid hormone response became normal and resulted in a normal thyroid image in the two dogs re-evaluated with gamma camera imaging. Daily treatment with oral levothyroxine (20 micrograms/kg) resulted in complete remission in puppies (n = 4) treated prior to 4 months of age. The other puppy failed to attain normal breed standards for height.(ABSTRACT TRUNCATED AT 250 WORDS)
