Phosphorylation – Oxidative, Protein, and Glucose

In chemistry and biology, phosphorylation is adding a phosphate group (PO₄^3-) to an ion or molecule. Phosphorylation is a reversible process. The removal of a phosphate group is dephosphorylation. Phosphorylation occurs in both prokaryotic and eukaryotic cells. It is important for energy storage and production, enzyme activation, post-translational modification of proteins, cell signaling, and many other processes.

Types of Phosphorylation

Phosphorylation is a common type of condensation reaction. Three of the most important forms of phosphorylation are glucose phosphorylation, protein phosphorylation, and oxidative phosphorylation.

Glucose Phosphorylation

Glucose and other sugars undergo phosphorylation as the first step of carbohydrate catabolism. Glycolysis begins with the conversion of D-glucose into D-glucose-6-phosphate. One function of glucose phosphorylation is regulating blood glucose concentration. Glucose is small enough that it readily permeates cells, but the phosphorylated molecule is too big. Also, glucose concentration relates to glycogen formation, which stores energy for later use. Cardiac growth also relies on glucose phosphorylation.

Protein Phosphorylation

Protein phosphorylation is an important post-translation modification. In other words, the phosphorylation occurs after a protein is translated from an RNA template. The reaction occurs primarily on side chains of serine, tyrosine, or threonine residues in eukaryotes and histidine in prokaryotes. These amino acids have nucleophilic (-OH) group that accepts the terminal phosphate group from ATP. The magnesium ion (Mg²⁺) chelates the other phosphate groups, making the transfer of the terminal phosphate more favorable. Kinases are responsible for phosphorylation, while phosphatases mediate dephosphorylation.

One type of protein phosphorylation is the phosphorylation of histones. Histones are proteins that associate with eukaryotic DNA and form chromatin. Histone phosphorylation occurs around damaged DNA. This opens up space around the broken sequence for repair mechanisms.

DNA repair is only one example of a cell using phosphorylation to change protein conformation. A conformation change affects a protein’s catalytic activity, essentially activating or deactivating enzymes. Also, conformation changes regulate binding of neighboring proteins. Cells use this for signal transduction.

Oxidative Phosphorylation

Cells use oxidative phosphorylation for storing and releasing chemical energy. In eukaryotic cells, oxidative phosphorylation occurs within mitochondria in the electron transport chain and chemiosmosis. Redox reactions transfer electrons to proteins and other molecules in the inner mitochondrial membrane. This releases energy, which chemiosmosis uses to make adenosine triphosphate or ATP.

Specifically, NADH and FADH₂ deliver electrons to the electron transport chain. The electrons transition from higher energy to lower energy states during their progression along the chain, releasing energy. Some of this energy pumps hydrogen ions (H⁺) and produces an electrochemical gradient. The end of the electron transport chains sees electrons transfer to oxygen. Oxygen bonds with H⁺ and forms water. The protons or H⁺ ions are the energy source that ATP synthase uses to make ATP. ATP dephosphorylation cleaves the terminal phosphate group and releases chemical energy in a form the cell can use.

During subsequent phosphorylation steps, adenosine forms AMP, ADP, and ATP. But, phosphorylation occurs with other bases. For example, guanosine phosphorylation forms GMP, GDP, and GTP.

Functions of Phosphorylation

Phosphorylation is a key process for cells. Its functions include:

• Cells use it to transmit signals to other cells.
• The energy-requiring and energy-releasing reactions maintain homeostasis.
• Cells use it to store and release chemical energy.
• It regulates enzyme and other protein functions.
• It moderates signal transduction pathways.
• Histone phosphorylation is a key step in DNA repair.
• Phosphorylation regulates the cell cycle, including cell growth and apoptosis.
• Because of this, understanding phosphorylation may unlock our understanding of aging.

Detecting Phosphorylation

Scientists apply several methods to detect and study phosphorylation, including mass spectrometry, immunoassays, kinase activity assays, electrophoresis, isotope labeling, and fluorescence,

References

• Campbell, Neil A.; Reece, Jane B. (2005). Biology (7th ed.). San Francisco, California: Benjamin Cummings. ISBN 0-8053-7171-0.
• Kresge, Nicole; Simoni, Robert D.; Hill, Robert L. (2011). “The Process of Reversible Phosphorylation: The Work of Edmond H. Fischer”. Journal of Biological Chemistry. 286 (3).
• Mitchell P., Moyle J. (1967). “Chemiosmotic hypothesis of oxidative phosphorylation”. Nature. 213 (5072): 137–9. doi:10.1038/213137a0
• Sharma, Saumya; Guthrie, Patrick H.; Chan, Suzanne S.; Haq, Syed; Taegtmeyer, Heinrich (2007). “Glucose Phosphorylation Is Required for Insulin-Dependent mTOR Signalling in the Heart”. Cardiovascular Research. 76 (1): 71–80. doi:10.1016/j.cardiores.2007.05.004
• Wikstrom M, ed. (2005). Biophysical and Structural Aspects of Bioenergetics (1st ed.). Royal Society of Chemistry. ISBN 0-85404-346-2.