Cytochromes Hydrogen & Electrons

By now you probably know that the Universal Energy Currency or Molecule is ATP. Adenosine Triphosphate is found in all organisms and this is a common link between organisms and evidence of a common ancestor.

The ETC operates by means of cytochromes.

• A cytochrome is a carrier for electrons
• These are cytochrome a, cytochrome b and cytochrome c
• Two cytochromes contain iron and one cytochrome contains copper
• Other cytochromes are involved in hydrogen carrying
• These cytochromes carry hydrogen from NADH₂ & FADH₂ 

Here is an outline of the involvement of hydrogen in the ETC:-

At the beginning of the ETC, NADH₂ is the reduced form of NAD. It has gained hydrogen ions. NAD is a hydrogen acceptor. Reduced NAD is a means of rephosphorylation of ADP > ATP.

At the beginning of the ETC, FADH₂ is the reduced form of FAD. It too has gained hydrogen ions. FAD is also a hydrogen acceptor. Reduced FAD is a means of rephosphorylation of ADP > ATP.

Succinate is involved in the reduction of FAD > FADH₂.

• NADH₂ & FADH₂ are hydrogen acceptors that need cytochromes that are hydrogen carriers rather than an electron carriers
• These cytochromes do the same job as the electron carriers because as hydrogen is bounced between their cytochromes, a small amount of energy is released
• The energy released by passing hydrogen atoms can rephosphorylate ADP > ATP. 
• Electrons released from this first part of the ETC. These are used in the second part of the ETC.
• The ‘spare’ H⁺ ions left over from the first part of the ECT go to combine with oxygen to form water

Here is the sequence of cytochromes b, c, & a and, in outline, their involvement in the ETC:

• Cytochrome b contains iron Fe³⁺ but the electron it picks up converts the iron to Fe²⁺. The iron is in a Haem, a prosthetic group
• Cytochrome c picks up an electron bounced from cytochrome b.
• Cytochrome c has Fe²⁺. As it bounces an electron to cytochrome a, the iron in cytochrome c converts back to Fe³⁺
• Cytochrome a contains not iron but copper in the form of 2Cu²⁺. Cytochrome a is in fact two cytochromes in one:- a + a₃. This is known as cytochrome oxidase.
  • As each electron is bounced from cytochrome to cytochrome, energy is released to attach a free phosphate group to ADP (rephosphorylation).

Cytochrome oxidase promotes reduction of oxygen to water.