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Summary. People with genetic haemochromatosis (GH) accumulate iron from excessive dietary absorption. In populations of northern European origin, over 90% of patients are homozygous for the C282Y mutation of the HFE gene. While about 1 in 200 people in the general population have this genotype the proportion who develop clinical haemochromatosis is not known. The influence of HFE genotype on iron status was investigated in 10 556 blood donors. The allele frequencies of the C282Y and H63D mutations were 8´23% and 15´3% respectively. Heterozygosity for C282Y occurred in 1 in 7´9 donors, for H63D in 1 in 4´2 donors, and 1 in 42 were compound heterozygotes. Homozygosity for H63D occurred in 1 in 42 donors and 1 in 147 (72) were homozygous for C282Y. Mean values increased for transferrin saturation (TS) and serum ferritin (sFn), and decreased for unsaturated iron binding capacity (UIBC) in the order: donors lacking the mutations, H63D heterozygotes, C282Y heterozygotes, H63D homozygotes, compound heterozygotes and C282Y homozygotes, but serum ferritin (sFn) concentrations were no higher in H63D heterozygotes and C282Y heterozygous women than in donors lacking mutations. The percentage of donors failing the screening test for anaemia or of those with sFn , 15 mg/l did not differ among the genotype groups. C282Y and H63D heterozygotes and donors homozygous for H63D were at no greater risk of iron accumulation than donors lacking mutations, of whom 1 in 1200 had both a raised TS and sFn. The risk was higher for compound heterozygotes (1 in 80, P 0´003) and for C282Y homozygotes (1 in 5, P , 0´0001). There was no correlation between sFn and either age or donation frequency in C282Y homozygotes. None of the 63 C282Y homozygous donors interviewed showed physical signs of overload or were aware of relatives with haemochromatosis. The Welsh Blood Service collects blood from about 140 000 people each year including an estimated 950 who are homozygous for HFE C282Y. They are probably healthy and unaware of any family history of iron overload.

The shikimate pathway is the biosynthetic route that is responsible for the production of essential aromatic compounds (1). These include the aromatic amino acids tryptophan, tyrosine, and phenylalanine, folic acid, an essential cofactor for many enzymatic processes, and salicylate, used for the biosynthesis of the siderophores through which bacteria acquire iron (2). The pathway is found in microorganisms and plants and has more recently been discovered in apicomplexan parasites (3, 4). The pathway is absent in higher organisms, making the enzymes of this pathway attractive as targets for the development of antimicrobial agents. Recent gene disruption studies have shown that operation of the shikimate pathway is essential for the viability of Mycobacterium tuberculosis (5), the causative agent of tuberculosis, a disease that remains a significant world-wide health risk (6). Although effective anti-tuberculosis drugs exist, the long treatment times required, the problems of latent or persistent tuberculosis (7), and the proliferation of multidrug-resistant strains of M. tuberculosis (8) have all created an urgent need for the development of new antimycobacterial agents.The first committed step in the shikimate pathway is the stereospecific aldol reaction between phosphoenolpyruvate (PEP) 4 and erythrose 4-phosphate (E4P) to produce 3-deoxy-Darabino-heptulosonate 7-phosphate (DAH7P), catalyzed by the enzyme DAH7P synthase (Fig. 1). DAH7P is converted into chorismate, the product of the main shikimate pathway, via six further enzyme-catalyzed reactions. At this point the pathway to the aromatic amino acids branches, with chorismate converted either to anthranilate by anthranilate synthase or to prephenate by chorismate mutase. Anthranilate ultimately produces Trp, whereas prephenate is converted into Phe and Tyr.As the first enzyme, DAH7P synthase, is a major control point for shikimate pathway flux. Several organisms express two or more isozymes of this enzyme that show different sensitivity to the pathway end products. Escherichia coli and Neurospora crassa each produce three isozymes, with each enzyme individually inhibited by either Phe, Tyr, or Trp (9,10

12 Centers for Disease Control. Classification system for human T-lymphotropic virus type Illymphadenopathy-associated virus infections. Ann Intern Med 1986;105:234-7. 13 Centers for Disease Control. Revision of the case definition of acquired immunodeficiency syndrome for national reporting. MMWR 1985;34: 374-5. 14 Peto K, Pike MC, Armitage DR, et al. Design and analysis of randomized clinical trials requiring prolonged observation of each patient. II. Analysis and examples.

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Abstract3-Deoxy-D-arabino-heptulosonate 7-phosphate synthase (DAH7PS) catalyzes the first step in the shikimate pathway, the pathway responsible for the biosynthesis of the aromatic amino acids Trp, Phe, and Tyr. Unlike many other organisms that produce up to three isozymes, each feedback-regulated by one of the aromatic amino acid pathway end products, Mycobacterium tuberculosis expresses a single DAH7PS enzyme that can be controlled by combinations of aromatic amino acids. This study shows that the synergistic inhibition of this enzyme by a combination of Trp and Phe can be significantly augmented by the addition of Tyr. We used X-ray crystallography, mutagenesis, and isothermal titration calorimetry studies to show that DAH7PS from M. tuberculosis possesses a Tyr-selective site in addition to the Trp and Phe sites, revealing an unusual and highly sophisticated network of three synergistic allosteric sites on one enzyme. This ternary inhibitory response, by a combination of all three aromatic amino acids, allows a tunable response of the protein to changing metabolic demands.

Background: Two enzymes from Mycobacterium tuberculosis involved in aromatic amino acid biosynthesis form a heterooctameric complex. Results: Complex formation boosts the catalytic activity of both enzymes and greatly extends the allosteric effector sensitivity. Conclusion: Enzyme interactions allow complex allosteric machinery of one of the complex partners to be shared. Significance: Sophisticated allosteric responses are delivered through protein-protein interactions, allowing enhanced metabolic control.