Below is a table of standard reduction potentials for reactions important to bio

Question

Below is a table of standard reduction potentials for reactions important to biological systems. Use this to answer the following questions.

Eo'

2H+ + 2e- --> H2

-0.42

NAD+ + 2H+ + 2e- --> NADH + H+

-0.32

So + 2H+ + 2e- --> H2S

-0.27

FAD + 2H+ + 2e- --> FADH2

-0.22

SO42- + 8H+ + 6e --> So + 4H2O

-0.20

NO3- + 2H+ + 2e- --> NO2- + H2O

0.42

NO2- + 8H+ + 6e- --> NH4+ + 2H2O

0.44

Fe3+ + e- --> Fe2+

0.77

O2 + 4H+ + 4e- --> 2H2O

0.82

Part A. Thiobacillus denitrificans couples oxidation of elemental sulfur to reduction of nitrate. Write the two half reactions that describe this process.

Part B. Would this occur under aerobic or anaerobic conditions? Explain why this is the likely case.

Part C. Would these microbes likely be autotrophs or heterotrophs?

Part D. This microbe has a very long generation time, on the order of several hours. Give two reasons why this microbe grows so slowly.

Eo'

2H+ + 2e- --> H2

-0.42

NAD+ + 2H+ + 2e- --> NADH + H+

-0.32

So + 2H+ + 2e- --> H2S

-0.27

FAD + 2H+ + 2e- --> FADH2

-0.22

SO42- + 8H+ + 6e --> So + 4H2O

-0.20

NO3- + 2H+ + 2e- --> NO2- + H2O

0.42

NO2- + 8H+ + 6e- --> NH4+ + 2H2O

0.44

Fe3+ + e- --> Fe2+

0.77

O2 + 4H+ + 4e- --> 2H2O

0.82

Explanation / Answer

A. For both denitrification and nitrate respiration, transcription of the key enzyme, nitrate reductase, is usually inhibited in the presence of O2 and derepressed by anoxia. In denitrification, derepression of nitrate reductase is followed by sequential derepression of the genes for the three other enzymes responsible for the overall reaction below (nitrite reductase, nitric oxide reductase, nitrous oxide reductase):

H2S + HS- + NO3- + CO2 + HCO3- + NH4+ --> SO42- + N2 + C5H7O2N (biomass) + H+ + H2O

5[CH2O]+4NO3+4H+?5CO2+2N2+7H2O.

Although N2 is typically the end product of denitrification, the intermediates NO and N2O often diffuse out of cells in small quantities and contribute to the concentrations of these important gases in the atmosphere. In addition, some de-nitrifiers, including fungal de-nitrifiers, do not possess nitrous oxide reductase and produce only nitrous oxide. In general, de-nitrifiers can use a wide range of organic substrates, but certain catabolic reactions that require oxygenases do not take place. Also, a variety of inorganic substrates can be respired using nitrate or nitrite as electron acceptor. Thiobacillus denitrificansthus oxidizes sulfide through denitrification.

B. Anaerobic respiration is defined as the oxidation of organic or inorganic substrates for ATP synthesis by oxidative phosphorylation using exogenously derived terminal (or alternate) electron acceptors other than oxygen. Energy and ATP yields are always lower than those obtained using molecular oxygen. The list of alternate electron acceptors is extensive, and includes the oxidized forms of metals (Fe3+, Mn4+), the oxyanions of many elements (e.g., arsenate, nitrate, nitrite, perchlorate, selenate, sulfate, and uranate) and some organics (e.g., dimethylsulfoxide, fumarate and trimethylamine oxide)

C. Autotrophs

It is the first and only autotrophic bacterium reported to oxidize U(IV) oxide to U(VI) oxide minerals anaerobically, which could partially counteract efforts to remediate uranium contaminated aquifers by in situ reductive immobilization (the radioactive and mobile U(VI) can be dissimilatorily reduced to the immobile form U(IV) by some microorganisms, such as Geobacter sp.).

T. denitrificans is one of the best studied of few obligately chemolithoautotrophic microorganisms which can combine inorganic sulfur-compound oxidation with denitrification.

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