Substrate level vs. Oxidative Phosphorylation

We use the term oxidative phosphorylation to describe how 2 molecules of FADH2 and NADH (produced in the Citric Acid cycle) are used to make ATP. We use the term “oxidative” because oxygen accepts an electron while the gradient made by the movement of electrons powers the creation ATP.

But why do we even need oxygen?

Can’t the electrons just float around the cell? Well they actually do sometimes. This is known as electron leakage. Normally the electrons will bind with oxygen and hydrogen to form stable molecules of water. If electrons leak from the transport chain before they reach complex IV they can form superoxides.

Superoxides are part of a larger group of reactive oxygen species(ROS) that are unstable. To stabilize themselves they will strip the valence electrons from the outer shell of nearby molecules. If this occurs on DNA molecules it can lead to permanent mutations that will ultimately cause the cell to undergo apoptosis.

An even worse case scenario is the formation of a cancerous cell. The cell does have a mechanism to deal with this. The enzyme superoxide dismutase can pick up these leaked electrons.

Substrate-level Phosphorylation

Oxidative Phosphorylation of the Electron Transport Chain(ETC) is in contrast to ATP made during glycolysis which is known as substrate-level phosphorylation. We use the term substrate level phosphorylation because ATP formation in glycolysis is powered by the addition/removal of phosphate groups (phosphorylation) to molecules of glucose (the substrate). Hence the term substrate-level phosphorylation.

An example of substrate level phosphorylation that occurs in glycolysis is the production of ATP when 1, 3 bisphophoglycerate is converted into 3-phosphoglycerate. The phosphate is removed directly from the substrate( 1, 3 bisphophoglycerate) and joined with ADP to make ATP. This process can occur in conditions where no oxygen in present.