Electron Transport Proteins

Question 1

What are cytochromes

Answer 1

Fe bound to a haem plus two other ligands - employ the Fe(III)/Fe(II) couple
Act as a 1e- redox shuttle

Question 2

What are Type 1 or “Blue Copper” centres?

Answer 2

Cu bound to a cysteine, two imidazoles and one (or two) other ligands - employ the Cu(II)/Cu(I) couple

Question 3

What are iron-sulfur (Fe-S) centres

Answer 3

• Involving FeS₄ tetrahedral as
• 1Fe, 2Fe-2S, 3Fe-4S, or 4Fe-4S centres
• Employ the Fe(III)/Fe(II) couple
• (With some sharing of the redox change between Fe’s)

Question 4

Redox change at the d-transition metal centre leads to…

Answer 4

… minimal structural reorganisation
and/or
The electron is added/lost from an orbital that is metal ligand non-bonding (doesn’t point directly at the ligand)

Question 5

What is a similarity between Cytochrome b and c

Answer 5

They are both six coordinate
(putting two amino acid residues on haemoglobin makes these cytochromes low spin)

Question 6

Protein X-ray crystal stuctures show …… change in geomtry between Fe(II) and Fe(III) forms in cytochrome
And why?

Answer 6

• Protein X-ray crystal stuctures show little change in geomtry between Fe(II) and Fe(III) forms in cytochrome c
• This is because cytochrome will be low spin (6-coordinate)
• Electron change is in the dxy orbital
• This orbital lies in between ligands, hence little structural change and rapid electron transfer

Question 7

What techniques could we use to example cytochrome?

Answer 7

Unpaired electron - EPR
Also use X-ray absorption spectroscopy to look at the oxidation state

Question 8

What are the biological uses for Type 1/Blue copper proteins?

Answer 8

• Plastocyanin in the higher plants - Transports electrons between photosystem I and II - the biological apparatus used in photosynthesis
• Azurin in bacteria - Transports electrons to cytochrome c oxidase

Question 9

Where does the blue colour come from in Type 1/blue copper proteins

Answer 9

• Blue colour comes from LMCT from electrons on SCys to Cu
• The structures of the Cu centre are essentially identical in the Cu(II) and Cu(I) forms
• This allows for minimisation of the reorganisation energy

Question 10

Why does the protein environment present a ligand set that is a compromise between those favoured by Cu(I) and Cu(II)

Answer 10

• Cu(II) is hard while Cu(I) is soft
• SCys + SMet are soft, while NHis + NHis are hard
• Compromise between hard and soft ligands and acids
• AND Cu(II) favours: 6,5,4 coordination while Cu(I) favours 2,3,4 coordination
• Both compromise on a 4 coordination
• (This allows for no real change in geometry between Cu(II) and Cu(I), allowing from rapid e- transfer)

Question 11

The ligands in Type 1/blue copper protein are…

Answer 11

fixed by the protein backbone
It is putting the copper in an Entatic state

Question 12

Iron-Sulfur centres occur in all living systems; they are considered to be primordial redox and catalytic centres
What are the similarities between these 4 centres

Answer 12

All Fe-S centres involve the Fe atoms (s) bond to a tetrahedral array of 4 S atoms; the S may be a cysteinyl residue of the protein of S²⁻ (Sulfide)

Question 13

All the centres, whether they contain 1,2,3, or 4 atoms, act as one-electron redox centres
The Fe atoms can be as Fe(II) or Fe(III) or the redox change may be shared by two or more Fe atoms - fractional oxidation state
How does this occur?

Answer 13

[Fe(III) (Cys)₄]⁻¹ + e⁻ ⇌ [Fe(II)(Cys)₄]²⁻
The additional electron goes into the dz² orbital which does not point towards a ligand - little change in geometry
Leading to rapid electron transfer

Question 14

The following molecule is a 4Fe-4s Ferredoxin[ ] ²⁻⁄ ³⁻
What is its geometry and how does it exchange electrons?

Answer 14

• Each Fe centre is attached to 3 other sulfurs and 1 SCys
• Hence the 4Fe-4S
• These centres only handle one electron at a time (1e- change, between a -1, -2 or -3 state)
• 1- charge is called a High potential iron protein (HiPIP)

Question 15

The one electron change is not localised on one Iron in 4Fe-4S Ferredoxin
How do we know this

Answer 15

• The charges on the Iron centres are not intergers
• This also allows for rapid electron transfer

Question 16

What is the protein environment like for Ferredoxin [Fe₄S₄(Cys)₄]²⁻⁄ ³⁻ ?

Answer 16

• 4Fe-4S Ferredoxin centres are strongly hydrogen bonded to the protein and employ the [Fe₄S₄(Cys)₄]²⁻⁄ ³⁻ couple
• The hydrogen bonds to the sulphur stabilise the accumulation of negative charge, making the redox potential more positive
• Allows for stabilisation of higher charges in a hydrophilic environment

Question 17

What is the protein environment like for HiPIP [Fe₄S₄(Cys)₄]¹⁻⁄ ²⁻

Answer 17

• In a HiPIP protein, a hydrophobic environment about the 4Fe-4S centre favours the [Fe₄S₄(Cys)₄]¹⁻⁄ ²⁻ the couple that uses centres of lower charge

Question 18

The redox potential of these Fe-S centres ranges from -750 to +360 mV (vs SHE); the value is determined by the nature of the Fe-S centre and the environment about the centre provided by the protein
Why has the HiPIP got its name?

Answer 18

Because the redox potential of the HiPIP protein is positive

Question 19

Work out the oxidation state change for HiPIP
[Fe₄S₄(Cys)₄]⁻ + e⁻ → [Fe₄S₄(Cys)₄]²⁻

Answer 19

Question 20

If we take a ferredoxin [Fe₄S₄(Cys)₄]²⁻⁄ ³⁻ and place it in a very hydrophobic environment, which oxidation state is this going to favour
What does this do to redox potential

Answer 20

[Fe₄S₄(Cys)₄]³⁻
It has a higher charge and hence is going to interact with the water more
This causes redox potential (Eθ) to become more positive
(protein environment controls the redox potential)

Question 21

How could the protein environment control the redox potential?

Answer 21

An environment of alkyl or aryl amino acids side chains where there may be less hydrogen bonding the the Sulphur atoms resists the accumulation of change, making the redox potential more negative