Energetic Balance of the Total (and I mean Total) Oxidation of a Fatty Acid with an Odd number of Carbons.

In previous posts we have discussed how the fatty acids with an odd number of carbons chain, release 1 unit of Acetyl CoA and 1 unit of Propionyl CoA, instead of the two Acetyl CoA units released when a fatty acid with an even number of carbons is beta-oxidized.

 

Let’s use some examples, (we represent here just the carbons in the chains):

 

Example 1: a fatty acid with 6 carbons (Hexanoic acid)

 

C-C-C-C-C-C-C

 

During the Beta-oxidation, three units of Acetyl CoA are released (two carbons each):

 

C-C/C-C/C-C

 

Example 2: A fatty acid with 7 carbons (Heptanoic acid):

 

C-C-C-C-C-C-C

 

During the Beta-oxidation two units of Acetyl CoA and one unit of Propionyl CoA are released (two units of two carbons and one unit of three carbons):

 

C-C-C/C-C/C-C

 

As discussed previously in another post, the Acetyl CoA are oxidized in the Krebs Cycle, but the Propionyl CoA is used in the formation of Succinyl CoA, in a process that consumes 1 ATP (-1 ATP).

 

The Succinyl CoA can continue in the Krebs Cycle and form Oxalacetate. Oxalacetate will react with Acetyl CoA (Citrate Synthase reaction) to form Citrate, following the reactions in the Krebs Cycle. If it happens, we can consider that the atoms of carbons of the Propionyl CoA have followed an anaplerotic pathway (to be used in the Kreb’s Cycle without being consumed). 

 

BUT these carbons could also be completely oxidized if they follow this sequence of reactions:

 

Succinyl CoA + GDP + (P) – -> Succinate + CoA + GTP (it is equivalent to 1 ATP)

 

Succinate + FAD – - – - >Fumarate + FADH₂ (It generates 2 ATP in the Respiratory Chain)

 

Fumarate + H2O—–> Malate

 

But the Malate can now diffuse from the matrix through the mitochondrial membranes and be decarboxylated (under the action of the cytoplasmatic malic enzyme) to Pyruvate (and production of 1 CO2).

 

Pyruvate can return to the interior of the mitochondria,  where another decarboxylation occurs, this time under the action of the  Pyruvate dehydrogenase complex, (with production of another CO2) and the formation of Acetyl CoA, whose Acetyl group will be oxidized in the Cycle producing other two molecules of CO2.

 

We can see that through this sequence of reactions it is possible the total oxidation of the three original carbons of the Propionate to 3 molecules of CO2! (To avoid confusions, observe that, yes, there are 4 decarboxylations, but one of the CO2 does not come originally from the Propionyl CoA, but from the carboxylation process in the conversion of Propionyl CoA to Succinyl CoA)

 

Which would be the energetic balance of the total oxidation of an odd chain fatty acid considering this sequence of reactions?

 

Let’s see:

 

Propionyl CoA to Succinyl Co A = -1 ATP

 

In the mitochondria, (following the reactions of the Kreb’s Cycle up to Malate):

 

Succinyl CoA + GDP + (P) –> Succinate +CoA + GTP (it is equivalent to 1 ATP)

 

Succinate + FAD ——– – - – > Fumarate + FADH₂ (It generates 2 ATP in the Respiratory Chain)

 

Fumarate + H2O————–>Malate

 

In the cytoplasm:

 

Malate + NADP⁺ - – - >Pyruvate + NADPH.H⁺ + CO2 (we will not consider this  reduced cofactor in the balance since NADPH.H⁺ is not a source of energy, but a source of reduction equivalents for synthetic reactions)

 

In the mitochondria again:

 

Pyruvate + CoA + NAD⁺ —-> Acetyl CoA + CO2 + NADH.H⁺ (Observe that this NADH.H⁺ is generated inside the mitochondria, so it yields 3 ATP)

 

The Acetyl CoA produced in the previous reaction, when oxidized in the Krebs Cycle: 12 ATP

 

Therefore, considering this metabolic way,

 

-1 +1 +2 +3 + 12 = 17 ATP as a result of  the total oxidation of the Propionyl CoA generated by the beta-oxidation of a fatty acid of odd number of carbons.

 

 

Therefore, for calculating the energetic balance we should add 17 ATPs from the oxidation of the Propionyl CoA, to the ATPs generated in the Beta-oxidation, and the ATPs generated as a result of the oxidation in the Krebs Cycle  of the Acetyl CoA units formed during the Beta-oxidation of the odd chain fatty acid. ( We should recall also that 2 ATPs are consumed in the initial activation of the fatty acid)

 

In our next post we will analyze the oxidation of the heptadecanoic acid (17 carbons) as an example of the application of these calculations.

 

Oxidation of a fatty acid with 17 atoms of carbon

(This post analise the energetic balance considering that the Propionyl CoA follows an anaplerotic fate)

 

Apply the equations described in the previous post:

 

N= Number of Carbons

 

(N/2) -1.5 = Number of rounds in Beta-oxidation

 

(N/2) -1.5 = Number of acetyl CoA produced in Beta-oxidation

 

So, in terms of production and consumption of ATP of ATPs, the oxidation of a 17-carbons fatty acid will show the following energetic balance:

 

Activation of a fatty acid to Acyl CoA = -2 ATP

 

Number of rounds in Beta-Oxidation:

 

(17/2) – 1.5 = 8.5 -1.5 = 7

7 rounds x 5 ATP/round = 35 ATP

 

Number of produced Acetyl CoA: 7 Acetyl CoA

 

7 Acetyl CoA x 12 ATP/Acetyl CoA = 84 ATP

 

Additionally, the Beta-oxidation has produced 1 Propionyl CoA. The conversion of Propionyl CoA to Succinyl CoA, as described in a former post, will consume 1 ATP (Consider -1 ATP).

 

As described in a previous post:

 

“We can consider the conversion of Propionyl CoA to Succinyl CoA as an anaplerotic pathway, in which case, the molecule of Succinyl CoA continue in the Krebs Cycle generating Oxalacetate in the following sequence of reactions:

 

Succinyl CoA + GDP + (P) —> Succinate + CoA + GTP (equivalent to 1 ATP)

 

Succinate   +  FAD ———-> Fumarate +  FADH₂ (Generates 2 ATPs in the Respiratory Chain)

 

Fumarate +     H2O————– > Malate

 

Malate +   NAD⁺ —————->  Oxalacetate + NADH.H⁺ (Generates 3 ATPs in the Respiratory Chain) “

 

Total = (-1+1+2+3) = 5 ATPs

 

Total of ATPs produced (considering the anaplerotic fate of the Propionyl CoA turned into Succinyl CoA) = -2+35+84-1+6 = 122 ATP                        

 

BUT…

 

We may also consider a total oxidation that would include the carbon atoms of the Propionyl CoA!

 

In our next post, we will consider what happens if, instead of the Succinyl CoA following an anaplerotic pathway, it follows a path that allow the carbon atoms of Propionyl CoA end up being oxidized to CO2. It would allow a real  total oxidation of all the original carbons of the heptadecanoic acid or any other fatty acid with an odd number of carbons.

 

 

Inhibitors of the Electron Transport Chain

 

Answer to Question B-09

 

As described in a former post, the inhibitors of the Electron Transport Chain are substances that bind to some of the components of the ETC blocking its ability to change in a reversible form from an oxidized state to a reduced state.

 

This inhibition results in the accumulation of reduced forms before the inhibitor point, and oxidized forms of the components of the ETC downstream (ahead) the inhibition point.

 

 

Since energy is not released, the synthesis of ATP also stops. The most important known inhibitors of the ETC are Amytal, Rotenone, Antimycin A, CO, Sodium Azide, and Cyanides.

 

Amytal, a barbiturate, and Rotenone, a plant product used as insecticide and pesticide, block the ETC between NADH dehydrogenase (Complex I) and CoQ.

 

Consequently, they prevent the utilization of NADH as a substrate. On the contrary, electron flow resulting from the oxidation of Complex II is not affected, because these electrons enter through QH₂, beyond the block.

 

The effect of Amytal has been observed in vitro, since the intoxication with amytal and other barbiturates in vivo affect mainly the CNS by acting on GABA-sensitive ion channels, an effect not related to the action of Amytal on Complex I.

 

Rotenone intoxications are very rare. In facts, some human tribes used to catch fishes by spreading plant extracts containing rotenone in the water, and this substance was easily absorbed by the fishes through the gills. These fishes were eaten later without notable side effects in humans, since rotenone is absorbed very difficult by the gastrointestinal tract. Usually, when taken in a concentrated form, irritating action in mucoses causes vomits.

 

It is interesting to note that Rotenone and MPTP (a neurotoxin), when administered in vein,  cause at the same time interference with the functioning of Complex I and a Parkinson-like disease. These substances affect primary neurons in substancia nigra; apparently the sequence is: impairment of Complex I, impairment of mitochondria metabolism, accumulation of free radicals, cell death, release of toxic compounds and destruction of other cells.

 

Antimycin A is an antibiotic produced by Streptomyces griseous that has been used as a piscicide for the control of some fish species. Antymicine A interferes with electron flow from cytochrome b_H in Complex III (Q-cytochrome c oxidoreductase).  In the presence of this substance, cytochrome bH can be reduced but not oxidized, consequently,  in the presence of antimycin A cytochrome c remains oxidized, as do the cytochromes a and a3 that are ahead.

 

Carbon monoxide (CO) is responsible for more than 50 % of death by poisoning worldwide. It is colorless and odorless; high levels can result from incomplete combustion of fuels: engine and furnace exhausts are important sources. Tobacco smoking increases CarboxyHb levels.

 

Carbon monoxide intoxication causes impaired oxygen delivery and utilization at the cellular level. The affinity of Hb for CO is almost 300 times higher than for Oxygen. An environment in which there is 100 ppm of CO is enough to form 16 % carboxyhemoglobin. The situation is worsen since the binding of CO to one of the Hem groups of Hemoglobin increases the affinity of the other three Hem groups for Oxygen, so the delivery of Oxygen to  tissues is very affected. The brain and the heart, that has a high Oxygen consumption, are the most affected. Myoglobin has even a greater affinity for CO than Hemoglobin. As a consequence of the binding of CO to these molecules, the heart functioning is very impaired and the patient presents sever hypotension. As described above, this intoxication is an important cause of death worldwide.

 

The affinity of respiratory chain components for CO is lower than for Oxygen,

but since the clinical status does not correlate very well with the carboxyhemoglobin levels, it is considered that the inhibition of Cytochrome Oxidase by CO also plays a role in CO intoxication. CO binds to the reduced form of iron in Hem groups (Fe++) in cytochrome Oxidase

 

On the contrary, in cyanide intoxication the inhibition of the respiratory chain has a primary role. Intoxication by cyanide can be seen relatively frequent in patients with smoke inhalation from residential or industrial fires. Also in persons related professionally with cyanide or derivatives in certain industries.  Intentional poisoning can be seen in suicidal persons with access to cyanide compounds. Cyanide affects practically all metalloenzymes, but its principal toxicity derives from the binding to the Fe+++ in the Hem groups in cytochrome Oxidase, inhibiting the functioning of the Electron Transport Chain. As a consequence, redox reactions in the respiratory chain will stop, energy will not be released, proton pumps will not function, so they will not return through Complex V, and the production of ATP will cease (Related question here).

 

Azides have an action on the respiratory chain very similar to cyanide, inhibiting the Hem groups of cytochromes in Cytochrome Oxidase (Complex IV). Azides are used as propellants in airbags, in detonant (explosive) industry and as preservative of sera an reagents. Some cases of azide intoxication in humans have been reported.

 

You can find more information about these inhibitors of the Electron Transport Chain in these links:

 

Antimycin A: toxicity, ecological toxicity and regulatory information

 

 

Risk assessment for Piscicidal Formulations of Antimycin

 

Leybell, I: Toxicity: Cyanide

 

Cyanide poisoning

 

 

Azide Toxicity

 

Sodium Azide Toxicity effects

 

Shochat, G.N.: Toxicity, Carbon Monoxide