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Bioinorganic Chemistry > Lecture 3 > Flashcards

Lecture 3 Flashcards

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1
Q

What is the problem of iron metabolism?

A
  • the average human contains 4.3 g of Fe and only loses it due to blood loss
  • Fe has low bioavailability and is toxic when unbound
  • Bioavailability is low as Fe(II) is insoluble at a neutral pH
  • Most minerals contain Fe (III) but it forms an oxygen bridged polymer
  • An excess of free iron is dangerous as it can cause radical reactions
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2
Q

What are siderophores and what organisms use them?

A
  • Siderophores are used in iron transport in simple organisms such as bacteria, fungi and yeast.
  • Fe 3+ is precipitated as Fe(OH)3 in aqueous media
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3
Q

How do siderophores work?

A

They are secreted from cells into the external medium where they bind to Fe 3+to give a soluble complex that can reenter a cell using a specific ATP driven receptor type

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4
Q

What are the two main groups of siderophores?

A

Hydroxamates and Catecholates

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5
Q

Describe the binding of Fe in siderophores

A
  • Fe is in high spin states with Oh coordination
  • Very strong chelate effect (k can be up to 10^52)
  • Fe complexes are var labile so studies often use Cr3+ instead to give kinetically stable complexes
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6
Q

How is the Fe released inside the cell?

A

1) Fe(III) is reduced to Fe(II), reducing the stability constant and triggering release
2) Ligand hydrolysis occurs in the cytoplasm, resulting in release
3) An intracellular iron-binding ligand strips it from the siderophore (possibly by a redox reaction)

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7
Q

What are transferrins and what are they used for?

A

Used in more complex organisms for the storage and transport of iron

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8
Q

Describe the structure of transferrins

A
  • glycoproteins (consisting of proteins and carbohydrates)
  • very high binding constants for Fe 3+ (10^22-26)
  • found in blood plasma, egg white and milk
  • molecular weight of 80 kDa
  • two similar but separate binding sites, around 10 A below the protein surface
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9
Q

Describe the bonding of Fe 3+ in a Tf active site

A

In each site Fe 3+ is coordinated by:

  • one carboxylate O- (Asp-)
  • two phenol ate O- (Try-)
  • an N imadazole (His)
  • Two O from bound carbonates to form an Oh centre
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10
Q

Describe the structural change in Tf as Fe binds

A

Tf is made of a high proportion of alpha helix, so is flexible.

As Fe binds a hinged motion occurs which allows the transferrin to recognise Fe-Tf but not Apo-Tf

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11
Q

How well do different ions bind to Tf?

A

Fe(III) binds much more strongly than Fe(II) as it is a hard ionic ligand

Al(III) binds well but is not easily released, which is the main reason for Al toxicity

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12
Q

What is TfR

A

The transferrin receptor in the cell membrane, which selectively binds to Fe-Tf, the first step to iron release in the cell

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13
Q

What are the steps for iron release in the cell?

A
  • Cell membrane forms vesicle initiated by Fe-Tf binding
  • pH lowered by membrane bound proton pump
  • release of Fe(III) as carbonates and Tyr protonation at low pH weakening binding to the Fe centre
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14
Q

What is Ferritin?

A
  • principal store of iron in animals and many plants
  • in humans it is mainly found in the liver, bone marrow and spleen
  • can hold up to 20% Fe by mass and is a non-toxic, water soluble method of storage
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15
Q

What are the 2 parts of Ferritin?

A

1) an Fe oxide mineral core

2) a protein shell 13nm in diameter with a 7.5 nm central cavity

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16
Q

Describe the mineral core in ferritin

A
  • 4500 high spin Fe centres
  • approximate composition of
    [FeO(OH)]8[FeO(PO4H2)]
  • iron oxide particles are coated with phosphate groups to anchor the iron oxide particles to the protien shell
17
Q

Describe the protein shell in ferritin

A
  • 24 sub units that link to make a hollow sphere
  • 3 fold axis pores are hydrophilic and lines with Asp and Glu
  • 4 fold axis pores are hydrophobic and lined with Leu
18
Q

What is the most likely mechanism for Fe transport in and out of the core?

A
  • Fe(III) is transported as Fe(II) along the hydrophilic pores probably as the Fe2+ aqueous ion
  • inside the cored oxidation can occur
  • Fe (III) is then reduced to release Fe(II) as required
  • backed up by experiment
19
Q

What experiment proves the mechanism for the transport of Fe(III) into ferritin

A

Treatment of Ferritin with a reducing agent and an Fe(II) chelating agent gives a ferritin.

Bioinorganic Chemistry (7 decks)

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