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;

D6.  Water Oxidation - The Kok Cycle

The mechanism by which water is oxidized occurs through a 5 stage Kok cycle illustrated below?

Figure:  Kok Cycle

Kok Cycle 

The Sn states in the Kok diagram denote different discrete oxidation states where n the number of oxidative “equivalent” stored in the OEC during cycle progression. 

What happens to Mn ions in the OEC during the cycle? What is required to initiate the next stage of the cycle starting from So? What is the role of Y161.+, an amino acid in PS2 that is very close to the OEC as shown in the figure below? 

Figure:  Y 161 and the OEC

Y161 and the OEC 

 

A new mechanism has been proposed based on the recent crystal structure.  A possible model for S3 is shown below with the distorted cubane structure and with an oxy and oxyl ligands.  Add a curved arrow to show how an oxygen-oxygen bond is made. 

Figure: Model for S3 of Kok Cycle

S3 of Kok Cycle

Given the maximal packing of ligands (including water) around the OEC, there must be a way for water to enter the site and for protons that are removed to move away from the complex and toward the lumen where they would contribute to the development of a proton gradient.

Tyr 161 is a critical amino acids that is situated proximal both to the OEC and to the source of electrons that reduce water.  The figure below shows the immediate environment around Y161 and the OEC.  Describe the role of the groups shown in the figure below.  An expanded view of the OEC and part of the protein is shown to the right.  Tyr 137 lies near the luminal interface of the protein complex. Describe the figure below and the role of the groups shown.

Figure:  Environment of Y161 in the OEC of T. vulcanus

EnvY161_OEC_Tvulcanus

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