BCH Graduate Survey of Biochemistry

Transcription

1 BCH 5045 Graduate Survey of Biochemistry Instructor: Charles Guy Producer: Ron Thomas Director: Marsha Durosier Lecture 55 Slide sets available at:

2 David L. Nelson and Michael M. Cox LEHNINGER PRINCIPLES OF BIOCHEMISTRY Fifth Edition CHAPTER 20 Carbohydrate Biosynthesis in Plants and Bacteria 2008 W. H. Freeman and Company

3 Basic flow of photosynthate (assimilated carbon) in plants Iodine staining of starch grains

4 Calvin Cycle of photosynthetic CO 2 fixation

5 The first step in the use of CO 2 is its condensation with a 5 carbon sugar ribulose 1,5-bisphosphate. A six-carbon intermediate is briefly formed, then it undergoes a cleavage to form two molecules of 3- phosphoglyceric acid. Ribulose 1,5-Bisphosphate + CO 2 (2) 3-Phosphoglyceric Acid This fixation of CO 2 is catalyzed by the enzyme ribulose-bisphosphate carboxylase-oxygenase, commonly referred to as "RuBisCO". It is a large and slow enzyme composed of eight each of two different subunits with a total molecular weight of about 550,000. It has a low turnover number, k cat = 3.3 s -1. This protein accounts for up to 50% of the soluble protein in a leaf, and sometimes it is said to be the most abundant protein on earth. RuBisCO is highly regulated, perhaps in part because of the central role it plays in photosynthesis and carbohydrate metabolism and as a consequence of the chemistry of the CO 2 fixation mechanism.

6 Structure of ribulose 1,5-bisphosphate carboxylase (rubisco)

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8 again. The reduction of a carboxyl group to an aldehyde cannot be accomplished by the action a reductase and a reducing agent alone. Instead, the group must first be activated before reduction can occur. This requires the expenditure of energy and the transfer of a phosphate from ATP. The phosphate ester bond can be reductively cleaved using the reducing potential of NADPH to yield the aldehyde, NADP + and P i. The conversion of a -COOH group to a -CHO group represents one of the classic reactions of metabolism, one that is seen over and over

9 Regeneration of ribulose 1,5- bisphosphate begins with the action of triose phosphate isomerase (same as the glycolytic enzyme only in a different part of the cell) which simply isomerizes glyceraldehyde 3-phosphate to dihydroxyacetone phosphate. Next one molecule each of dihydroxyacetone phosphate and glyceraldehyde 3-phosphate undergo a aldol condensation to form fructose 1,6-bisphosphate (gluconeogenesis).

10 The activity of transketolase (pentose pathway) which transfers 2C units from a donor to an acceptor is required. In this case, fructose 6- phosphate is the donor and glyceraldehyde 3-phosphate is the acceptor forming erythrose 4-phosphate and xylulose 5-phosphate (pentose pathway). Another aldol condensation occurs between erythrose-4-phosphate and dihydroxyacetone phosphate to form sedoheptulose 1,7- bisphosphate (pentose pathway) which is dephosphorylated at the C1 position to yield sedoheptulose 7-phosphate.

11 The action of transketolase transfers 2C from sedoheptulose 7- phosphate to glyceraldehyde 3-phosphate to form one xylulose 5- phosphate and ribose 5-phosphate (pentose pathway). Xylulose 5- phosphate is epimerized at the C3 position to form ribulose 5- phosphate, and ribose 5-phosphate is isomerized to ribulose 5- phosphate (pentose pathway).

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13 Finally ribulose 5-phosphate is phosphorylated at the C1 position to yield ribulose 1,5-bisphosphate the original acceptor for CO 2 (unique to Calvin Cycle). The overall cost of fixing CO 2 via the Calvin Cycle to produce one mole of hexose is: 6CO NADPH + 12H ATP + 11H 2 O F-6-P + 12NADP ADP + 17Pi Why F-6-P instead of glucose? This may seem like a great deal of energy to put into the synthesis of a molecule of hexose sugar, but often photosynthesis is capable of generating more than enough NADPH and ATP to drive its formation. What is generally most limiting photosynthesis for the majority of plants today is the availability of CO 2. This presents plants with a dilemma.

14 Finally ribulose 5-phosphate is phosphorylated at the C1 position to yield ribulose 1,5-bisphosphate the original acceptor for CO 2 (unique to Calvin Cycle).

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16 RuBisCO has a major biochemical flaw. As a result of its reaction chemistry, the reaction can proceed using oxygen instead of CO 2. This forms one molecule of 3-phosphoglyceric acid and one molecule of 2-phosphoglycollate. This leads to a process known as photorespiration and involves a metabolic pathway that leads to the production of CO 2. This process is considered unproductive because of the loss of CO 2 that has been previously fixed.

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