The Importance of ATP in Metabolism and How it’s Made

Have you even wondered what happens to the cheese burger you just ate or the soda you just drank? Your body is trying to extract whatever nutrition or energy it can get out of the cheese burger and soda. The process of obtaining nutrition and energy from our food is called metabolism and without it we would simply die. More clearly metabolism is the breaking down and building up of biological molecules. One of the most important molecules that we use and make in metabolism is ATP. ATP is short for adenosine triphosphate and is the main energy source for our bodies. ATP was discovered in 1929 by a German chemist named Karl Lohmann. It wasn’t until ten years later that a United States scientist, Fritz Lipmann, found that ATP was the body’s main source of energy. To the right is the structure of ATP where the top structure is the basic molecular structure, middle is showing what ATP looks like 3D, and the bottom is showing how much space each atom is using.

Function of ATP in Metabolism

As stated earlier, ATP is involved in several reactions that take place in all life forms. ATP helps transport molecules within the cell and makes reactions more favorable. To provide an example, I will look at two steps in one of the most well known pathways of metabolism, glycolysis. Glycolysis is the breakdown of glucose to get energy. In the first step of glycolysis, (shown below) a phosphate from ATP is added is to glucose. This phosphate prevents glucose from passing through the hydrophobic cellular membrane and leaving the cell. Besides preventing molecules from leaving the cell, ATP also makes reactions more favorable. For example, when breaking fatty acids down ATP adds a phosphate making it more energetically favorable. The reaction is more energetically favorable because a phosphate leaving a molecule is more stable than an hydroxyl group (OH-).

Synthesis of ATP

Clearly without ATP our bodies cannot function. ATP is constantly being recycled in the body, but there is also a need to make new ATP. An enzyme called ATP synthase generates ATP in the mitochondria. Within the mitochondrial membrane is found ATP synthase. To the right is an image of ATP synthase. It is composed of two portions F­1 and Fo. F1 sits outside of the membrane and contains 5 subunits—3a:3b:1g:1d:1e. The three b subunits bind to the substrate, either AMP or ADP. The complex mechanism of ATP synthase was discovered over a period of ten years from the 1960’s through the 1970’s by Paul Boyer. He found that the three active sites in ATP synthase change their binding affinity for the reactants: ATP, ADP and phosphate. According to Dr. Antony Crofts from the University of Illinois at Champaign-Urbana, “ The change in affinity accompanies a change in the position of the g-subunit relative to the a, b-ring, which involves a rotation of the one relative to the other. In the direction of ATP synthesis, the rotation is driven by a flux of H+ down the proton gradient, through a coupling between the g-subunit, and the c-subunit of FO.”

This proton gradient is generated in the electron transport chain, shown below. The H+ ions within the inner membrane space originated from the reduced form of NAD+ and FADH (NADH and FADH2). NADH and FADH2 are generated from the breakdown of the foods we eat, like the cheeseburger and soda. Once the concentration of the H+ ions is greater in the inner membrane space in comparison to the outer membrane, some H+ must leave the inner membrane space. The only way they can enter the outer membrane is by going through ATP synthase. The flow of H+ from the inner membrane to the outer membrane through ATP synthase is what causes the release of ATP from the active site. It is important to note that H+ is not required for ATP synthesis, but in fact only its release from the enzyme.

Final Note

ATP is one of the most important molecules in the body. In metabolism it helps with transport and making reactions more energetically favorable. ATP synthesis is fueled by the foods we eat. This is why it is most especially important to have proper breakfast everyday or at least on exam days.

References

“18.4 A Proton Gradient Powers the Synthesis of ATP — Biochemistry — NCBI Bookshelf.” A Proton Gradient Powers the Synthesis of ATP. 2002. Web. 06 Nov. 2010. <http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=stryer∂=A2528&gt;.

Crofts, Antony. ATP Synthase. 1996. Web. 6 Nov. 2010. <http://www.life.illinois.edu/crofts/bioph354/lect10.html&gt;.

Nelson, David L., and Michael M. Cox. Principles of Biochemistry. 5th ed. New York: Freeman, 2008. Print.