A: More questions about Bioenergetics (B-03 to B-05)

Original Questions

Answer to Bioenergetics Question B-03

                                                                       

Answer: (d)

 

Answering this kind of questions is very easy. It just requires recalling the composition and organization of the Respiratory Chain:

 

 

 

NADH.H+ is accumulated. Observe that reductions equivalents (Hydrogens) from NADH.H+ enter the electron transport chain through Complex I (NADH dehydrogenase). It means that Complex I is not working properly.

Hydrogens from FAD can not enter the electron transport chain either. It means that Complex II (succinate dehydrogenase) and other enzymes related to the Electron Transport Chain hat use FAD as cofactor can not be dehydrogenated.

 

Coenzyme Q (Ubiquinone) is also accumulated in a reduced form.

 

Which compound is in an oxidized state? Cytochrome c.

 

It is obvious then that there is a problem between CoQ, that remains reduced, and Cytochrome c, that remains oxidized.  (Of course, since Coenzyme Q remains reduced, it can not accept more electrons from FADH2 or from NADH.H+ and these cofactors remain in the reduced state.)

 

Observing the organization of the electron transport chain, we can say that this new inhibitor probably is affecting Complex III.

 

Antimycin A is known to block the transfer of electrons between two components of Complex III, (Cytocrom b and c1), so its effect would be very similar to the effects described in the question for the new inhibitor.

 

 

Answer to Bioenergetics Question B-04:

 

Answer (e)

 

Once again, observe the organization of the respiratory chain.

 

X represents Complex III

 

 

Answer to Bioenergetics Question B-05:

 

Answer: (e)

 

ATP synthase is called Complex V of the Respiratory chain (observe the position in the graphic).

During the spontaneous flow of electrons through the ETC (from the component with the most negative to the component with the most positive redox potential), energy is released. This energy is used to pump protons from the mitochondrial matrix to the intermembrane space.

 

This accumulation of protons in the intermembrane space creates an electrical potential (positive in the outside of the internal membrane) and a pH gradient (outside the internal membrane is more acid, since there are more free H+).

 

These protons tend to return spontaneously to the matrix, driven by the electrical and the chemical gradient that have been created through their pumping across the membrane.

 

ATP synthase synthesize ATP from ADP and (P) using the energy released from the spontaneous return of protons from the intermembrane space to the matrix.

 

That is why it is said that the energetic coupling between the ETC and the Oxidative Phosphorylation is indirect.

 

Notes:

 

Sometimes the terms Electron Transport chain and Respiratory chain are use indistinctly. However, a more strictly use restricts the term Electron Transport Chain to the components of the Respiratory chain responsible of the transport of reduction equivalents from reduced cofactors to Oxygen, forming water, and releasing energy in the process.

 

Oxidative Phosphorylation is the process of biosynthesis of ATP using the energy that is released in the respiratory chain. (This energetic coupling is indirect, as described before)

 

The concept of Respiratory chain then would include the concepts of Electron Transport Chain and Oxidative Phosphorylation, and should be described as the set of reactions through which reduction equivalents are transferred from reduced cofactors to Oxygen releasing energy that is used for the biosynthesis of ATP.

 

The integration between the Electron Transport Chain and the Oxidative phosphorylation, forming the Respiratory chain, is clearly seen in this short movie

 

This animation also illustrates this relationship:

Voet, Voet, Pratt: Fundamentals of Biochemistry

The coupling of Electron Transport chain and ATP Synthesis

 

 

A: (B-02), Bioenergetics: Arsenic poisoning and Enzymes of the Central Metabolic Pathway

Original Question

 

 

 

Answer: (e)

 

Both enzymes (in fact, multienzymatic complexes) require Lipoic acid for their function.

 

Lipoic acid goes between oxidized and reduced forms during the reaction:

 

Lipoic acid

 

 

 

Dihydrolipoic Acid

 

 

Arsenic, mainly the trivalent form, reacts with the thiols groups (-SH) of dihydrolipoamide (dihydrolipoic linked to the enzymatic protein). Consequently, the interconversion between the reduced and the oxidized form of lipoic acid is not possible. It affects very important multifunctional enzymatic complexes.

 

 

Pyruvate dehydrogenase, alpha-keto glutarate dehydrogenase and branched chain amino acid dehydrogenase are very similar multienzymatic complexes that include lipoic acid in their structure, and have in common the following characteristics:

 

1. – All of them catalyze the global reaction:

 

Alphaketoacid  +  CoA + NADH.H⁺ ————>  acyl CoA + NAD⁺  + CO2

 

2.- All of them are formed by three enzymes that participates directly in the reaction and  five coenzymes:

           

Enzymes: A Ketoacid Decarboxylase

                               A Dihydrolipoyl Transacylase

                               A Dihydrolipoyl Dehydrogenase

 

            Cofactors: TPP, CoA, Lipoic acid, FAD and NAD.

 

3.- The mechanism of the reaction (and the Delta G) is the same

 

This graphic represents the reaction catalyzed by the complex of the alphaketoglutarate (or 2 oxo glutarate) dehydrogenase. Observe that the ability of oxidation and reduction of the thiols groups of lipoic acid is  key in the reaction.

  

 

 

 

As a consequence of the inhibition of the activity of these enzymatic complexes, pyruvate can not become acetyl CoA, the main feeder of Krebs cycle, and alphaketoglutarate can not continue to succinyl CoA in the Cycle. It affects deeply the production of energy in the cell, the membrane permeability becomes altered,  and Pyruvate, lactate and other metabolites accumulates. Other proteins and cell enzymes are also affected by the binding of Arsenite to thiols groups. Other forms of Arsenic can affect other processes in a different way.  Eventually, neurological disturbances appear and a multiorgan failure can occur that lead the person to death.

 

Arsenic poisoning can be accidental, occupational, suicidal or  homicidal.

 

More than 100 million people are at risk of this intoxication because of excessive arsenic in drinking water

     

  

Bangladesh is one of the regions where people is more exposed to arsenic in water. This song, describes how to avoid arsenic poisoning and explain misconceptions about the intoxication:

 

 

 

 

 

 

 

Children are a group at risk for accidental intoxication with rat poison containing arsenic.

 

The Borgias were a well know family that used Arsenic to solve discrepancies with other persons.

 

The death of Napoleon has been related to arsenic poisoning.

 

Some impressionist apparently developed Arsenic Intoxication due to the use of Emerald Green, a pigment based on Arsenic. It was Cezanne preferred pigment and Monet and Van Gogh used this pigment very frequently also. Cezanne’s Diabetes, Monnet’s blindness and Van Gogh’s disorders have been attributed to Arsenic intoxication as a consequence of the use of this pigment .

 

Paul Cezanne: Jas de Bouffan

 

 

 

More information about the toxic effects of Arsenic can be found in:

 

Marcus, S: Toxicity, Arsenic

 

 

 

Arsenic Toxicity: Physiologic effects

 

Question (B-02): Arsenic Poisoning and Enzymes of the Central Metabolic Pathway

Question (B-02), Bioenergetics: Arsenic poisoning and Enzymes of the Central Metabolic Pathway

 

Most forms of arsenic are toxic, but the arsenite form is much more toxic than others. Arsenite forms a stable complex with enzyme-bound lipoic acid. For the most part, arsenic poisoning is explained by inhibition of those enzymes that require lipoic acid as a coenzyme. So, two important enzymes inhibited as a consequence of arsenic poisoning are:

 

a)     Citrate synthase and Pyruvate dehydrogenase

b)     Citrate synthase and Isocitrate dehydrogenase 

c)      Citrate synthase and alpha-keto glutarate dehydrogenase

d)     Isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase

e)     Pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase.

f)       Pyruvate dehydrogenase and isocitrate dehydrogenase

 

 

Answer

 

BIOENERGETICS PUZZLE:

 

Can you find the metabolites of the Krebs Cycle in this puzzle?