Iron Acquisition in Bacteria

Question 1

Why is iron essential for nearly all life forms?

Answer 1

Due to its redox properties (Fe2+ <-> Fe+ cyclic valence changes in oxidation state), making it essential for:
- Electron transport
- DNA synthesis
- Metabolic enzymes

Question 2

What are the extracellular concentration of iron in mammals?

Answer 2

Around 10nM

Question 3

What concentration of iron do bacteria need for optimal growth?

Answer 3

100-1000nM

Question 4

What are the 3 broad groups of iron-requiring enzymes?

Answer 4

• Non-haem mono/di-iron enzymes
• Haem enzymes
• Iron-sulfur cluster enzymes

Question 5

What is an example and function of a non-haem mono/di-iron enzyme?

Answer 5

Dehydrogenases- function in central metabolic pathways

Question 6

What are some examples and functions of haem enzymes?

Answer 6

• Catalase (preventing oxidative stress by breaking down H2O2)
• Cytochrome oxidase (ETC in respiration)

Question 7

What are some examples and functions of iron-sulfur cluster enzymes?

Answer 7

• Succinate dehydrogenase (respiration)
• ABCE1 (translation)

Question 8

What is the global transcriptional regulator for genes in iron metabolism?

Answer 8

The Fur regulon

Question 9

What does Fur stand for?

Answer 9

Ferric uptake regulator

Question 10

What does the Fur regulon do in iron-replete (lots of iron) conditions?

Answer 10

When iron is abundance:
- Fur binds Fe2+, adopting an active conformation.
- It then binds to Fur boxes in promoter region
- This blocks RNA polymerase, repressing transcription of iron acquisition genes§

Question 11

What does the Fur regulon do in iron-deplete (lots of iron) conditions?

Answer 11

When iron is lacking:
- Fe2+ is no longer bound to Fur, releasing it from the Fur boxes
- This allows transcription of iron acquisition genes

Question 12

What are 4 different iron acquisition systems used by bacteria?

Answer 12

• Siderophores
• Feo system
• Transferrin receptors
• Haem uptake

Question 13

What are siderophores?

Answer 13

Low-molecular-weight chelators with extremely high affinity for Fe2+

Question 14

What is an issue with iron acquisition?

Answer 14

Iron cannot diffuse across membranes- must be actively transported

Question 15

How does iron exist in aerobic environments?

Answer 15

In its oxidised form, Fe3+

Question 16

What is an issue with Fe3+?

Answer 16

It has a very low solubility, so difficult to acquire

Question 17

What are the properties of Fe2+?

Answer 17

• Soluble
• Very low availability
• Toxic at high concentrations

Question 18

Why must iron acquisition be tightly regulated?

Answer 18

Excess iron is toxic; must be regulated in bacteria by Fur regulon

Question 19

What bacteria use siderophores?

Answer 19

Most G - and some G +

Question 20

What is the source of iron for siderophores?

Answer 20

Free Fe3+

Question 21

What is the E. coli siderophore called?

Answer 21

Enterobactin

Question 22

How does enterobactin uptake iron?

Answer 22

• Enterobactin secreted by E. coli bind Fe3+
• Enterbactin-Fe3+ complex recognised by specific outer membrane receptors
• Transported across membrane; iron released intracellularly

Question 23

What is the outer membrane receptor for E. coli siderophore enterobactin?

Answer 23

FepA

Question 24

What is the periplasmic shuttle for E. coli siderophore enterobactin?

Answer 24

FepB

Question 25

What is the inner membrane ABC transporter for E. coli siderophore enterobactin?

Answer 25

FepDGC

Question 26

How is imported iron released for use?

Answer 26

Degraded in cytoplasm by esterase (Fes) to release iron for use

Question 27

What is siderophore piracy?

Answer 27

Some bacteria also express receptors for other species' siderophores; allows them to "steal" iron from other species

Question 28

What is a host counter-measure to siderophores, and what is pathogen evasion mechanism to this?

Answer 28

Host counter measure: - Lipocalin-2 binds to catecholate siderophores, preventing bacterial uptake Evasion: - Some pathogens have evolved stealth siderophores resistant to Lipocalin-2

Question 29

How does quorum-sensing play into siderophores?

Answer 29

Production of siderophores is triggered by unbinding of Fur, and modulated by quorum sensing

Question 30

What is the Feo system?

Answer 30

Ferrous iron uptake system

Question 31

Where is Fe2+ found in the body?

Answer 31

Only available in acidic, low-oxygen environments (like the gut)

Question 32

What is an example of a bacteria that uses Feo system?

Answer 32

Helicobacter pylori

Question 33

What is the key protein for Feo system?

Answer 33

FeoB protein, which forms a membrane channel for Fe2+

Question 34

What are transferrin and lactoferrin?

Answer 34

Proteins that bind and transport iron in the body. Only contain around 1% of iron in the host, but are not locked inside cells (like haem)

Question 35

Where do transferrin and lactoferrin transport iron?

Answer 35

Transferrin - circulates in blood Lactoferrin - secretions like tears, saliva and mucosal surfaces

Question 36

What bacteria use direct iron acquisition from transferrin?

Answer 36

Gram - bacteria like: - Neisseria meningitidis - Haemophilus influenzae

Question 37

What is TbpB?

Answer 37

Surface lipoprotein that captures transferrin, and transfers it to TbpA

Question 38

What is TbpA?

Answer 38

A cell membrane receptor of transferrin, that (following interaction with TonB complex) extracts Fe3+ from transferrin and transports it into the periplasm

Question 39

What happens after the iron is passed into the periplasm?

Answer 39

Iron binds to FbpA, which delivers it to the FbpBC transporter Iron can them enter the cytoplasm through the FbpBC transporter

Question 40

What is the role of TonB complexes in iron acquisition?

Answer 40

Provides energy for active transport of iron complexes (siderophores/transferrin/hem) across the outer membrane

Question 41

What does S. aureus use to uptake haem?

Answer 41

Isd System

Question 42

How does the lsd system work?

Answer 42

1. S. aureus surface proteins IsdB and IsdH bind to haem or haem complexes 2. Haem is passed through a series of Isd proteins to IsdF, and ABC transporter 3. Transported into cytoplasm, where the haem is broken down by IsdI and IsdG to release usable iron

Question 43

What are different haem sources?

Answer 43

- Free haem - Haemoglobin - Haem complexes

Question 44

What are three host strategies to prevent pathogens from acquiring iron?

Answer 44

- NRAMP1 - Lipocalin 2 - Hepcidin

Question 45

What is NRAMP1?

Answer 45

Natural Resistance-Associated Macrophage Protein 1

Question 46

Where is NRAMP1 found, and what does it do?

Answer 46

Found in phagocytic cells (especially macrophages). It transports Fe2+ (and Mn2+) out of phagosomes, depriving intracellular bacteria of metal ions

Question 47

What is an example of pathogen that NRAMP1 affects (w/ evidence)?

Answer 47

Mycobacterium tuberculosis, which resides in macrophages. Polymorphisms in NRAMP1 are linked to increased susceptibility to TB

Question 49

What is hepcidin?

Answer 49

A hormone made by the liver that controls systemic iron levels

Question 50

How does hepcidin reduce iron availability?

Answer 50

- Inhibits absorption from the gut - Blocks release from iron-storing cells

Question 51

What is a pro and con hepcidin?

Answer 51

- Helps prevent infection by starving pathogens of iron - Cause of anaemia when chronic infections are present (persistent high levels of hepcidin)

Question 52

What is NRAMP1, Lipocalin 2 and Hepcidin all examples of?

Answer 52

Nutritional immunity