And domain ii disulfide bridges carbohydrate

Info icon This preview shows pages 27–31. Sign up to view the full content.

and domain II (residues 91-251); (_) disulfide bridges; (*) carbohydrate attachment site. See Reference 39.
Image of page 27

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

20 1 I TRANSITION-METAL STORAGE, TRANSPORT, AND BIOMINERALIZATION Earth grew in a reducing atmosphere, in which the iron was substantially more available because it was present as ferrous-containing compounds. In contrast to the profoundly insoluble ferric hydroxide, ferrous hydroxide is relatively sol- uble at near neutral pH. It has been proposed that this availability of iron in the ferrous state was one of the factors that led to its early incorporation in so many metabolic processes of the earliest chemistry of life. 6,38 In an oxidizing environ- ment, microorganisms were forced to deal with the insolubility of ferric hydrox- ide and hence when facing iron deficiency secrete high-affinity iron-binding compounds called siderophores (from the Greek for iron carrier). More than 200 naturally occurring siderophores have been isolated and characterized to date. 42 Most siderophore-mediated iron-uptake studies in microorganisms have been performed by using cells obtained under iron-deficient aerobic growth condi- tions. However, uptake studies in E. coli grown under anaerobic conditions have also established the presence of siderophore-specific mechanisms. In both cases, uptake of the siderophore-iron complex is both a receptor- and an energy- dependent process. In some studies the dependence of siderophore uptake rates on the concentration of the iron-siderophore complex has been found to conform to kinetics characteristic of protein catalysts, i.e., Michaelis-Menten kinetics. For example, saturable processes with very low apparent dissociation constants of under one micromolar (l fLM) have been observed for ferric-enterobactin transport in E. coli (a bacterium), as shown in Figure 1.12. Similarly, in a very 80 c "E 60 complex concentration (J..lM) Figure 1.12 Effect of MECAM analogues on iron uptake from E. coli. Iron transport by 2 f.LM ferric enterobactin is inhibited by ferric MECAM. OJ E m ""6 E Eo Q) Q) C""- al Q. ::J Q) LL en en 40 20 -5 2 4 6
Image of page 28
II. BIOLOGICAL SYSTEMS OF METAL STORAGE, TRANSPORT, AND MINERALIZATION 21 different microorganism, the yeast Rhodoturala pilimanae, Michaelis-Menten kinetics were seen again with a dissociation constant of approximately 6 JLM for the ferric complex of rhodotoroulic acid; diagrams of some representative siderophores are shown in Figure 1. 13. The siderophore used by the fungus Neurospora crassa was found to have a dissociation constant of about 5 JLM and, again, saturable uptake kinetics. o;: l OH ~ N ~ CH3 S / N S~COOH pyochelin mycobactin P pseudobactin UOH yOH enterobactin Figure 1.13 Examples of bacterial siderophores. See Reference 42.
Image of page 29

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

22 4 [Fe(ent)P- (or analogue) \ I \ I tepA protein receptor ~~==:t:I:::::::::::::::::::::::=============~~o:uter membrane . periplasmic space cytoplasm leakage ot [Fe (ent)] 3- to solution Figure 1.14 Model for enterobactin-mediated Fe uptake in E. coli.
Image of page 30
Image of page 31
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern