Shoelaces and Uncoupled Respiration

The world is a beautifully complex place. There’s hardly ever just one way to complete a task. Take tying shoes for example; the battle still rages on whether they most effective way is utilizing the bunny ears method or just the one loop technique. Either way, the job gets done. Biochemistry is no exception to this rule. Many biological systems have more than one way to accomplish the same process. Thermogenesis has been defined as the process of producing heat. The human body does this in efforts to maintain its internal temperature homeostasis. In their recent article, authors Jonathan Long et al. uncovers PM20D1, a new enzyme that leads to another method of undergoing thermogenesis. Before understanding how and why PM20D1 becomes involved with this process, its predecessor was the starting material.

The most researched thermogenesis pathway deals with uncoupling protein 1 (UCP1). This protein is present in two types of adipose tissues, brown fat and beige facts (Long). UCP1 disrupts the natural proton gradient established by oxidative phosphorylation. The protein pumps protons back into the mitochondrial matrix without producing ATP, but producing heat in the process (Letica). This process is known as uncoupled respiration (Fig. 1). In addition to expression of UCP1, scientists continually test the many different roles these specific adipose tissues may be involved in. The first step of this study investigates other possible pathways of undergoing uncoupled respiration. Long et al. suspect co-expression of another gene along with Ucp1. The research team proceed to compile a list of what they refer to as “core thermogenesis” genes along with what genes are expressed in the same conditions as UCP1. This is how peptidase M20 domain containing 1 (PM20D1) becomes the focus of this study. With a signal peptide and no transmembrane domains, PM20D1 fits the hypothesized secreted enzyme. To confirm this, tagged Pm20d1 gene is transfected into cells and found to be expressed in both the cell cultures as well as the in the media. With that data supporting the idea PM20D1 is a secreted enzyme, the authors move on to determining the effects of the enzyme on a live biological system and transfects mice. While on a high fat diet, the mice expressing the PM20D1 show a decrease in weight gain compared to the mice transfected with GFP as a control. The difference in weight is due to different percentage of fat mass compared to lean mass. Complimentary to the weigh finds, a calorimetry measurement show that mice with PM20D1 has a higher energy spending despite the same exact diet between the PM20D1 mice subjects and control GFP mice subjects. Both results indicate that this enzyme is the cause of these findings.

After seeing that PM20D1 is likely to influence biomass regulation and energy exertion, the authors now have to determine what exactly is occurring with the enzyme. With knowing that PM20D1 is part of the mammalian M20 peptidase family and how this family tends to conduct activity on small molecule substrates, the authors use liquid chromatography mass spectrometry to determine PM20D1 molecular effects in the blood. Mice injected with the PM20D1 vectors show more peaks with a m/z of 428, which corresponds to the molecular ion of N-oleoyl phenylalanine. Tandem mass spectroscopy supports that peak’s identity by showing a m/z peak of 164 that corresponds to a phenylalanine ion. The authors use mass spectroscopy tools to determine the presence of N-acyl amino acids in PM20D1 mice rather than the GFP mice.

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