Re: During electron transport in chlorophyll photosynthesis, what are Fd,Pq,Pc?

Hi Lauren,

This is actually a very good question. At first, I thought that the answer 
should be easy to find in any decent Biochemistry textbook.  Then I looked 
in mine. Pq, Pc, and Fd are all abbreviations for molecules that ferry 
electrons between different compounds. The reason that they are drawn in 
circles in your book is that they are recycled in the process. 

Pq stands for plastoquinone, just one member in a family of compounds known 
as quinones. One member of this family is so widely found that it was named 
ubiquinone (from ubiquitous). Plastoquinone is a lipid soluble compound 
found diffused in the thylakoid membrane of chloroplasts in plants. It 
binds reversibly to photosystem II and accepts electrons that have been 
energized by light. It has a ring structure with two ketone groups, one on 
each end. When it takes up electrons the ketone groups are reduced to 
alcohol groups. In this form, quinones are called quinols. It also takes up 
two hydrogen ions. It then moves away from photosystem II to the cytochrome 
b6/f complex. There it donates its electrons to reduce cytochrome b6. In 
the process it is recycled to its original state as a quinone. The 
cytochrome b6/f complex then uses the energy of the electrons to pump 
protons across the thylakoid membrane. 

The electrons are then passed to Pc, or plastocyanin. Plastocyanin is a 
protein that can accept electrons by virtue of a coordinated copper ion 
that is reduced from Cu2+ to Cu+. Plastocyanin functions much like 
cytochrome c of the mitochondrial electron transport chain and indeed 
substitutes for it in some photosynthetic bacteria and algae. Plastocyanin 
then donates the electrons to the chlorophyll of photosystem I, which has 
been oxidized by a previous round of photosynthesis. 

Photosystem I then reenergizes the electrons, which have lost some of their 
spunk after all this shuttling and pumping of protons, and passes them to 
Fd, or ferredoxin. Ferredoxin is an iron-sulfur protein that diffuses 
around on the stroma face of the thylakoid membrane. It transfers the 
electrons to an enzyme called ferredoxin-NADP oxireductase. This enzyme 
uses the energized electrons to reduce NADP+ to NADPH. NADPH is a molecule 
that is used as a basic currency within the cell to carry out all kinds of 
different functions. The ferredoxin molecule is oxidized to its previous 
state by this and can go back and get some more electrons from photosystem 
I. 

If you want to know more about photosynthesis,  there is an excellent web 
site on it at http://www.life.uiuc.edu/govindjee/paper/gov.html

I am currently working in Dr. Whitmarsh's lab. He is one of the authors of 
this paper. Thanks for the question. It was an opportunity for me to 
refresh my knowledge of the details of photosynthetic electron transfer 
processes. 

Sincerely,
J. Todd Holland
Graduate student in Biophysics
At the University of Illinois at Urbana-Champaign
Fellow in the Integrative Photosynthesis Research Training Grant
Hollandtodd@hotmail.com