13. Arsenic Metabolism

Question 1

What is the oxidation states of the following: Arsenate, Arsenite and arsine

Answer 1

Arsenate - +5
Arsenite - +3
Arsine - -3

Question 2

Is arsenic toxic?

Answer 2

Yes but not the elemental form (0) as it is not soluble

Question 3

How is arsenate toxic?

Answer 3

Enters cells via phosphate transport system and replaces phosphate in ATP synthesis. Arsenic is not viable for life.

Question 4

How is arsenite toxic?

Answer 4

Enters cells via diffusion/glycerol transport system and inactivates proteins by binding to the thiol group (and inactivating it)

Question 5

How is elemental arsenic oxidised to arsenite then arsenate?

Answer 5

Arsenic to Arsenite - chemical oxidation

Arsenite to Arsenate - microbial oxidation (aerobic/anaerobic)

Question 6

How may arsenate be reduced to arsenite?

Answer 6

Microbially

Question 7

What is the equation for the oxidation of arsenite? What is the gibbs free energy of the reaction?

Answer 7

2H3AsO3 + O2 –> HAsO4 2- + H2AsO4- + 3H+

-175.8kJ/mol (at pH7)

Question 8

Place the oxidation of arsenite, iron and sulphur in order of how much energy they produce

Answer 8

most-sulphur-arsenite-iron-least

Question 9

How does the pH of the medium change when arsenite is oxidised?

Answer 9

pH decreases (similar way if you oxidised sulphur to sulphuric acid)

Question 10

What is thingies favourite arsenite oxidiser from the granites goldmine in australia?

Answer 10

NT26, its got two cute little villi

Question 11

What are the two types of arsenite oxidation?

Answer 11

Chemolithoautotroph (uses CO2 as energy source) and heterotroph (use organic matter for energy source)

Question 12

What does arsenite oxidase do? Describe it’s structure

Answer 12

Catalyses oxidation of arsenite to arsenate

Heterodimer/heterotetramer with 85/98kDa and small 15kDa subunits

Metalloenzyme - Mo, Fe, S redox cofactors

Question 13

How does the AiO enzyme work?

Answer 13

The molybdenum cofactor goes from VI to IV, catalysing a 2 electron transfer reaction.

The electrons are then transferred from molybdenum to 3Fe-4S (in alpha subunit) then to Rieske 2Fe-2S (in beta subunit)

Question 14

How does autotrophic arsenite oxidation occur?

Answer 14

Calvin cycle is used to fix CO2. NADH (generated by reverse electron flow) and ATP (from oxidation) is then generated.

As(III) oxidised to As(V), electrons passed to AroA then to AroB then Cyt C. From here the electrons can travel down two separate pathways. Either it travels through the bc1 complex to produce NADH or goes to aa3 cyt c oxidase to produce ATP

Question 15

How is arsenite (III) oxidase expression regulated?

Answer 15

Some organisms it is constitutive, others it is induced - As III acts as inducer

Question 16

Which parts of the AiO operon are constitutively expressed? Which may be induced?

Answer 16

Constitutively: AioX (binds AsIII - interacts with aioS), aioS (sensor histidine kinase - interacts with aioX), aioR (transcriptional regulator - induces transcription with RpoN), cyt c, moeA1 (Molybdenum biosynthesis)

Induced: aioB (Rieske 2Fe-2S cluster), aioA (Mo, 3Fe-4S)

Question 17

What are sigma factors?

Answer 17

Transcription initiation factors that enable specific binding of RNA polymerase to promoters. Different sigma factors are activated in response to different conditions. Sigma factors are distinguished by their molecular weight.

Question 18

Describe the process of AiO gene expression

Answer 18

As(III) activates aioX. aioX phosphorylates aioS. aioS autophosphorylates, then phosphorylates (by phosphate transfer) aioR. Sigma factor (encoded by rpoN) then binds to the promoter, RNA polymerase binds to sigma factor/promoter complex and bends RNA round, aioR binds upstream of the RNA, also binding to the RNA polymerase. This allows transcription of aioB, aioA, cyt C, moeA1.

Question 19

What are the applications of thr arsenite oxidase?

Answer 19

Can act as biosensor for As(III) - measure electric current to see how much arsenite in water.

Arsenite oxidised releases two electrons, mediator becomes reduced, electrons reduced and then detected by electrode which gives current. The current is proportional to the amount of arsenic.