The mitochondria, also called the “powerhouse” of the cells, is where ATP is made.
Cells use fats, carbohydrates and sometimes a little protein to create energy. These macronutrients are broken down in the cell to provide electrons for the electron transport chain, creating a mitochondrial membrane potential (Δψm). Complex I, III, & IV, powered by electrons, pump protons out of the mitochondria and create a proton gradient (Δp).
Δψm provides the driving force for ATP synthesis in the mitochondria. The higher the Δψm, the higher the energy capacity of the inner mitochondrial membrane, and the higher the potential for ATP synthesis.
The Δψm also controls the operation of the proton pumps.
Protons then re-enter the mitochondria through complex V, also called ATP synthase, which creates ATP.
Protons can also re-enter the mitochondria via proton leak, mostly through uncoupling proteins (UCPs). This process doesn’t generate ATP, but generates heat instead.
An increase in uncoupling reduces the Δp and mops up reactive oxygen species (ROS).
Without the electron transport chain (ETC), there will be no ATP synthesis.
Many, if not all, of the complexes and transporters in the mitochondrial membrane are stabilized by cardiolipin. The composition of cardiolipin is influenced by the type of fatty acids which are most abundant in the diet. A loss of cariodlipin, results in a loss of complex stability and efficiency.
Furthermore, fats are present in the membranes of the cells as well as in the mitochondrial membrane. They also influence the function of the mitochondria.
So how do different fats affect all of this?
You get three kinds of fats, saturated fats (SFAs), monounsaturated fats (MUFAs) and polyunsaturated fats (PUFAs).
SFAs have no double bonds, MUFAs only have one double bond and PUFAs have more than one. A double bond allows the fat to bend and be more flexible and fluid. PUFAs are also the most susceptible to the damage of a free radical. When PUFAs are damaged by a free radical it causes oxidative stress, inflammation and loss of function.
So where are most of the ROS created? In the ETC of course. The majority of ROS are created at complex I and III, but can also occur at any complex due to “electron leak”. The exit of electrons, prior to the reduction of oxygen to water at cytochrome c oxidase (complex IV), causes the production of superoxide. The membrane fluidity is most likely the biggest contributing factor which promotes the so-called “electron leak”; the more double bonds the PUFA contains, the more fluid the cell would be, and the more electrons would leak (increasing oxidative stress potential).
The ETC has two states. State 3 and 4.
State 4 is the resting state where succinic dehydrogenase (complex II) provides most of the electrons to the Q couple.
State 3 is when there is more input from pyruvate to complex I compared to complex II. The rate of electron flow, in the respiratory chain, is low in state 4 and greatly increased in state 3, because the reduction status of respiratory chain carriers, especially those of complexes I and III (NADH:NAD and ubiquinol:ubiquinone, respective), are higher in the resting state than in the active state. This explains why ROS production, by mitochondria oxidizing, mainly succinate (complex II), is much higher in the resting state than in the active state.
Resting state (state 4): ROS generation is relatively high when the mitochondria is oxidizing, mainly succinate (generating lots of FADH2 through complex II). This phenomenon can be related to reverse electron transfer (RET). The resting state has a higher Δψm than the active state, and is usually around 180 mV. Uncoupling proteins are usually higher during state 4. In short, state 4 can be characterized as a state without a need to produce much ATP (hence resting state), high ROS production and elevated uncoupling.
Active state (state 3): ROS generation is relatively low in mitochondria oxidizing substrates which generate NADH, such as pyruvate. The active state has a lower Δψm than the resting state, and is usually around 150 mV. In fact, a drop of Δψm by as little as 30 mV (that accompanies transition from State 4 to State 3) can decrease the rate of ROS generation by several-fold. Even a 10% decrease in Δψm by 20 mV decreases ROS production in heart mitochondria by about 60%. In short, state 3 can be characterized as a state with a high need for ATP, low ROS production and low uncoupling.
So let’s look at how different fats affect the ETC and ROS production.
The first study I want us to look at is this one:
“In PUFA deficient mitochondria, mitochondrial volume was enlarged (+45% in state 4 and two-fold in state 3).”
PUFA depletion results in bigger mitochondrias to work more efficiently. An increase in both states indicates less ability for electron leak to occur, less RET, less uncontrolled ROS production, and a greater ability to produce ATP.
“Taken together, these results are in agreement with both an increased non-ohmic proton leak and an increased redox slipping.”
The increase in non-ohmic proton leak indicates an increase in protons leaking back into the mitochondria, reducing the Δp. UCPs were not increased in this experiment, so the proton leak was through the basal proton leak and not through uncoupling proteins (UCPs). The basal leak is unregulated and correlates with metabolic rate.
The redox slipping is where electrons are transferred through the respiratory complexes, in particular cytochrome c oxidase, without pumping protons into the intermembrane space. This also reduces the Δp.
Also, according to the study, proton leak was more active in state 4, whereas redox slip happened in state 3.
One can view the ETC as a pipe. When the pipe is PUFA depleted, it is a big pipe (large in diameter), and with lots of PUFAs present, it’s a small pipe (small in diameter). A big pipe reduces the internal pressure and resistance, allowing more electrons to flow freely and reduce the risk of electrons leaking and making free radicals (such as superoxide and causing oxidative stress). Whereas a small pipe will have a lot of pressure, and electrons will be forced to leak out, thus, reverse electron transfer will take place more readily.
PUFA depletion protects the cells, by preventing electron leak, and also increasing proton leak which reduces ROS production.
↑ Proton leak = ↓ Δψm = ↓ ROS
The second study I want us to look at is this one here:
As you can see from state 4, complex I and III are mostly in the reduced state, that is why more ROS are produced. State 4 is most active when the mitochondria is running mainly on fats. Fats in the mitochondria are transported inside by the enzyme carnitine palmitoyltransferase I (CPT-1). It’s been found that unsaturated fat, but not saturated fats such as palmatic acid, have the greatest inhibitory effect on complex I & III.
Again, when PUFAs are consumed there will be more PUFAs present in the mitochondria compared to SFAs, which will inhibit the complexes in the ETC; generating more ROS and less ATP.
“PUFA were more potent inducers of O2·− generation than saturated or monounsaturated FFA, (ii) the stimulation of O2·− generation correlated with a partial inactivation of Complexes I and III”
and furthermore (emphasis mine)…
“FFA (free fatty acids, being PUFAs) might stimulate ROS production due the depletion of cytochrome c from mitochondria, thereby interrupting the electron flow from Complex III to Complex IV.”
“In fact, it was observed in the present work that unsaturated long-chain fatty acids, Lin (linoleic acid) and Ara (arachidonic acid), and the saturated branched-chain fatty acid Phyt stimulated ROS production, whereas saturated fatty acids Myr (myristic acid) and Pal (palmitic acid) were less active or without effect”
“FFA are … also inhibitors of the ADP/ATP exchange”
Remember from one of the previous posts, ADP/ATP exchange is the ANT complex, which is the rate limited transporter in getting ADP into the cell so that it can be converted to ATP.
So PUFAs increase ROS more than SFAs, by inhibiting complex I, III, ANT, inducing electron leak and inhibiting proton leak. PUFAs in the cardiolipin will be oxidized by the ROS, which will then release cytochrome c, leading to more ROS generation, and will further lead to apoptosis or nercosis, if there isn’t enough ATP available (which will most likely be the case).
PUFAs are bad news, and cause a pathological increase in ROS production. An acute increase in ROS production is useful as a messenger, but excessive ROS production is harmful. Excessive ROS damages the cell membranes, cardiolipin, DNA, free fatty acids and proteins present in the cells, and causes inflammation and cell death.
My advise is to keep your PUFA intake as low as possible, preferably below 4g daily.