Structural Biochemistry/Cell Organelles/Mitochondria/Cardiolipin and the Mitochondria

Overview
Cardiolipin is known as the signature phospholipid of mitochondria. It is responsible for a wide range of mitochondrial functions, including but not limited to ATP synthesis. Therefore, disorientation and disturbance in the metabolism of cardiolipin can cause pathological issues. Recently, there is a study on the enzymes that participate in cardiolipin biosynthesis and remodeling. After biosynthesis of cardiolipin, there are three remodeling enzymes that modify the acyl chain composition of cardiolipin, producing either a “dysfunctional” cardiolipin that is tied to michondrial dysfunction or a tissue-specific cardiolipin in its mature form. The following will discuss newly found molecules involved in cardiolipin metabolism as well as how human diseases are affected by the changes in cardiolipin metabolism.

Cardiolipin Background
Cardiolipin formation is key to the enzyme functionality of the mitochondria thereby with it malfunctioning mitochondria related diseases can occur. This compound is referred to as the “signature phospholipid” of the mitochondria because it is always present as well as is being formed there. It also appears to promote ATP formation which is the major role of the mitochondria.



The Phospholipid Uniquely in Mitochondria
When cardiolipin is present in an organelle’s membrane, that organelle must be a mitochondrion. This is the reputation of cardiolipin, the signature phospholipid of mitochondria. There, there is no surprise that most of the cardiolipin in a cell is closely tied to the inner membrane of mitochondria. Furthermore, the cardiolipin synthesis actually takes place in the inner membrane of mitochondria. Unlike other phospholipids that are also synthesized in a special compartment of the cell, cardiolipin remains closely associated with the mitochondrial membranes and does not get distributed out to the other endomembrane system like other phospholipids do. Because of this close association between cardiolipin and mitochondria, it is hypothesized that cardiolipin plays a huge role in ATP production through oxidative phosphorylation. In addition to this, since cardiolipin is only found in membranes that has an electrochemical gradient, it therefore also supports that cardiolipin has something to do with ATP production in the powerhouse of the cell. In addition to oxidation phosphorylation, cardiolipin also play a role in many mitochondrial activities. Cardiolipin contains two phosphatidyl groups connected by a glycerol. For this reason, cardiolipin is a lipid dimer that has four acyl chains instead of two as seen in other phospholipids. Out of all the biosynthetic enzymes except for cardiolipin synthase, they do not have any acyl chain specificity. Thus, the last acyl chain composition is not made during biosynthesis but rather in deacylation-reacylation/transacylation reactions. When added together, these reactions make the final form of cardiolipin cell/tissue-specific. In other words, different organisms can have different final molecular forms of cardiolipin and even in different tissues in the same organism. This proposes that the different forms of cardiolipin exist for the different tissues and cells function, mainly to accommodate the functions and energetic demands of the different tissues/cells. However, we are still unsure of whether having different acyl chain compositions will affect the function itself. We do know that the Barth syndrome is associated with the first inborn error of cardiolipin metabolism, since its cardiolipin is alternated in biosynthesis. So far, there are three known cardiolipin remodeling pathways as well as known proteins in the different stages of the cardiolipin biosynthetic pathways.

Recently found biosynthetic pathways
Cardiolipin biosynthesis takes place in the mitochondrion of eukaryotic cells. The enzymes of the dephosphorylation of phosphatidylglycerolphosphate and initiation of the cardiolipin remodeling cascade were recently found.

Phosphatidylglycerolphosphate phosphatase

The proteins Gep4 and PTPMT1 were identified in the phosphatidylglycerolphosphate phosphatases. Initially, Gep4 was detected in a screen in yeast as genetic interactors of prohibitions, which led to identifying Gep4 as a key player in the phosphatidylglycerolphosphate phosphatase of cardiolipin synthetic pathways. Indeed, a gep4 null has small amounts of cardiolipin and phosphatidylglycerol, which build up phosphatidylglycerolphosphate and destabilized respiratory supercomplexes. Furthermore, this is not able to grow on carbon sources that need a working OXPHOS system. In addition, recombinant Gep4 dephosphorylates phosphatidylglycerolphosphate to phosphatidylglycerol in experiments conducted in the laboratory.

Recently, the function of the substrates of PTPMT1 was discovered. PTPMT1 is part of the phosphatase and tensin homolog family that lives in the mitochondria. Knockout mices of PTPMT1 died in the uterus before embryonic day 8.5, showing that PTMPT1 is necessary for life. In observing the mouse embryonic fibroblasts of the PTPMT1 knockouts, slow growth, OXPHOS defects, reduced complex I levels, and altered inner membrane morphology were all seen. In addition, cardiolipin and phosphatidylglycerol levels decreased while phosphatidylglycerolphosphate accumulated. Like Gep4, recombinant PTPMT1 dephosphorylates phosphatidylglycerolphosphate to phosphatidylglycerol. While Gep4 is only present in plants and fungi, PTPMT1 is conserved evolutionarily. Still, these two proteins play a major role in the inner membrane of mitochondria and catalyze the same reaction. Interestingly enough, PTPMT1 is able to rescue a Geo4 null. Even though cardiolipin is present on both sides of the inner membrane, the two proteins Gep4 and PTPMT1 reside in the matrix face of the inner membrane only.

Cardiolipin phospholipases

The removal of a single acyl chain leads to monolysocardiolipin, which initiates cardiolipin remodeling. In yeasts, tafazzin (Taz1) is the transacylase that participates in cardiolipin remodeling. It does so by taking an acyl chain from another phospholipid and connecting it to the monolysocardiolipin. In the upstream of Taz1, cardiolipin deacylase 1 (Cld1) resides there. Surprisingly, Δcld Δtaz1 yeasts does not have monolysocardiolipin and has normal level of cardiolipin. However, Cld1 is observed to be involved in more than one metabolic pathways. This is due to the growth phenotype of the Δcld1 Δtaz1 strain is more disastrous than just one mutant. Secondly, Cld1 can add water to phospholipids as well as cardiolipin.

Formation Cardiolipin and Functionality
This molecule proceeds through a maturation process before becoming functional following the steps:

The pathway for forming Cardiolipin requires many enzymes, 1 for each step. Problems with Cardiolipin formation may result in lowered functionality of the mitochondria implying the following roles of the Cardiolipin in the mitochondria: Increasing efficiency of the OXPHOS- oxidative phosphorylation involved in ATP formation
 * Stabilized assemblies involved in this process
 * Traps protons resulting in changes in the gradient which is involved in the process that produces the most ATP in the cycle which results in an increasing gradient and a in turn a greater ATP production
 * Help in the process of cell death
 * Is key to other compounds being able to recognize and identify the mitochondria

An important aspect to proper activation of the Cardiolipin are the remodeling process of which there are three without which it does not function properly. There are three key enzymes that promote this remodeling process: The major diseases involved in the malfunctioning of these include: Wilson’s disease and Barth syndrome.
 * Tafazzin
 * Monolysocardiolipin acyltransferase 1
 * Acyl-CoA: lysocardiolipin acyltransferase 1

Conclusion
While the studies in cardioipin metabolism and how it can lead to human diseases is still new, there is a consistent effort to conduct experiments. So far, we have the complete biosynthetic pathway and remodeling inventory. For future steps, the regulation of cardiolipin metabolism and how it relates to health can be further studied. With the increasing interest and models in search of answers, we are close to accumulating more knowledge for this signature phospholipid of mitochondria.