Cellular Respiration (V) Oxidative Phosphate Addition

Oxidative Phosphorylation Steps

Picture_Oxidative Phosphorylation: Electron Transport Chain and Chemiosmosis

• High energy electron carrying NADH and FADH2 enter the Mitochondrial Matrix.
  • They come from the Krebs cycle and indirectly from Glycolysis.
• These electron carriers are Oxidized and pass their electrons down a series of proteins embedded in the mitochondrial membrane.
  • The pathway of these electrons is called the Electron Transport Chain. 
  • The process is driven by various enzymes.
• There are 6 proteins involved in this electron cascade,
  • 4 of which are considered complex and
  • consist of many protein subunits (Complexes I,II, III, IV). 
  • The remaining two, Q and Cytochrome C, are simpler proteins but are key in facilitating the movement of electrons. 
• As electrons move, they release energy, energizing Complexes I, III, IV to pump protons (H+) across the membrane.
  • The resultant proton gradient  drives the manufacture of ATP by Chemiosmosis of the protons. 
  • The Chemiosmosis drives the chemo-mechanical actions of the enzyme ATP Synthase which Phosphorylates ADP to ATP.  
• NADH and FADH2, after being Oxidized and giving up their electrons, are regenerated and head back to the Krebs cycle to repeat the process.
  • That is , NADH–>NAD+ and FADH2–>FAD
  • NAD+ and FAD are the oxidized forms of these molecules
• In the absence of oxygen (anaerobic), Pyruvate helps regenerate NADH to NAD+
  • When oxygen is scarce, NAD+ regeneration is limited in the electron transport chain.
  • In anaerobic conditions, pyruvate acts as an electron acceptor and generates NAD+ from NADH.
  • Lactic Acid is also produced.
  • This whole process is called Metabolic Fermentation  

Protein Complex Actions

Let’s go through the steps again with a focus on the protein complexes.  

• Electrons transfer through the protein complexes
  • They are complex in that they are composed of many types of protein units that assume various unique shapes. 
• Protein Complex I: Receives electrons from NADH (NADH is oxidized to NAD+).
  • It contains flavin mononucleotide (FMN – derived from Vitamin B2) and iron-sulfur (Fe-S)-containing proteins.
  • It helps to establish a hydrogen gradient (i.e. one side has more than the other side)
  • across the inner membrane by pumping hydrogen ions across the membrane. 
• Protein Complex II receives electrons from FADH2 and allows them to flow to Coenzyme Q.
  • It contains a Vitamin B2 derivative and iron-sulfur clusters as well 
  • Coenzyme Q (ubiquinone) connects the complex I and II electron flow.   
• Protein Complex III pumps protons to the intermembrane space and passes electrons to Cytochrome C.  
• Protein Complex IV accepts electrons from Cytochrome C
  • and is also energized to pump more hydrogen into the intermembrane space.
• Complex IV is the terminal electron acceptor and combines the electrons with oxygen to form water.

2H+ + 2e + 1/2 O2 ——-> H20

So, the electron cascade ultimately terminates when the electrons (along with hydrogen) react with oxygen to form water.

In other words, oxygen, the final electron acceptor in the Electron Transport Chain is reduced and forms water.  

Proton Gradient

The

• energizing of the electron transferring proteins
• and the subsequent pumping of protons into the intermembrane space of the Mitochondria,
• create a concentration gradient (higher concentration in the intermembrane space). 
• Since the protons are charged, an electrical gradient is also created.
• To imagine both phenomena, think of a rising potential energy that will be released if there is a path to a lower energy state.
• e.g. like water behind a dam. 

The pathway to release this gradient is the enzyme ATP synthase. 

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