Biochemistry Online: An Approach Based on Chemical Logic

Biochemistry Online

CHAPTER 8 - OXIDATION/PHOSPHORYLATION

D:  THE LIGHT REACTION OF PHOTOSYNTHESIS

BIOCHEMISTRY - DR. JAKUBOWSKI

04/15/16

Learning Goals/Objectives for Chapter 8D: 

After class and this reading, students will be able to

  • described the mechanistic similarities between mitochondrial oxidative/phosophorylation in which NADH and FADH2 are regenerated on reduction of O2 and the light reaction of photosynthesis in which O2 and a reducing agent, NADPH are produced;
  • describe similarities in fluorescence resonance energy transfer and exciton transfer;
  • describe the difference in properties between chlorophylls acting as antennae and chlorophylls at the reaction center;
  • describe how sunlight driven excitation of chlorophyll molecules at the reaction center produces an oxidzing agent strong enough to oxide water and form O2, itself a powerful oxidizing agent;
  • explain the general flow of electrons from dioxgen to NADP+ through a series of mobile and membrane protein bound electron carriers in the Z scheme of electron transport in the chloroplast thylacoid membranes;
  • explain with picture diagrams how oxidation of H2O and phosphorylation reactions (to produce ATP) are coupled in in the Z scheme;

D9.  Production of Hydrogen:  Hydrogenases (repeated from 8B)

Our world desperately needs an energy efficient way to produce H2 for energy production without producing waste pollutants.  Catalytic cracking of molecules and newly developed fuel cells offer two possibilities. Wouldn't it be great if a reactant like water could be used for H2 production (without the use of electrolysis) or expensive metal catalysts?  Nature may show the way.  Bacteria (even E. Coli found in our GI system) can use simple metals like iron to produce H2 from H+ with electrons for the reduction of H+ coming from a donor (such as a reduced heme in proteins): 

Dred+ H+ <=> Dox + H2

The reaction is also reversible in the presence of an acceptor of electrons from H2 as it gets oxidized:

Aox+ H2  <=> Ared + H+

The enzymes that catalyze hydrogen production are hydrogenases (not dehydrogenases).  Note that the name hydrogenases best reflects the reverse reaction when a molecule (P) in an oxidized state gets reduced (to S) and H2 gets oxided to H+. 

Crystal structures of hydrogenases show them to be unique among metal-containing enzymes. They contain two metals bonded to each other.  The metal centers can either be both iron or one each of iron and nickel.  The ligands interacting with the metals are two classical metabolic poisons, carbon monoxide and cyanide.  Passages for flow of electrons and H2 connect the buried metals and the remaining enzymes. The metals are also bound to sulfhydryl groups of cysteine side chains.  It appears that two electrons are added to a single proton making a hydride anion which accepts a proton to form H2.   In the two Fe hydrogenases, the geometry of the coordinating ligands distorts the bond between the two iron centers, leading to irons with different oxidation numbers.  Electrons appear to flow from one center to the other, as does carbon monoxide as well.  Ultimately, hydrogenases or small inorganic mimetics of the active site could be coated on electrodes and used to general H2 when placed in water in electrolytic experiments.

Photosynthesis - ASU

Photosynthesis on the Web - 2002

 

backNavigation

Return to Chapter 8D:  The Light Reactions of Photosynthesis Sections

Return to Biochemistry Online Table of Contents

Archived version of full Chapter 8D:  The LIght Reaction of Photosynthesis

 

Creative Commons License
Biochemistry Online by Henry Jakubowski is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.