Exam Review - Chapter 20

The starting material sixth molecule of triose

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Unformatted text preview: • Inner membrane of these plastids are impermeable to polar and charged molecules o Transport is mediated by transporters • CO2 assimilation occurs in three stages o 1st Stage: Carbon- fixation reaction: condensation of CO2 with a five- carbon acceptor, ribulos 1,5- bisphosphate to form two molecules of 3- phosphoglycerate o 2nd Stage: 3- phosphoglycerate is reduced to triose phosphate o 3rd Stage: five of the six molecules of triose phosphate are used to regenerate three molecules of ribulose 1,5- bisphosphate, the starting material Sixth molecule of triose phosphate is used to make hexose for fuel and other building materials o Overall process is cyclical Pentose phosphate pathway Reductive pentose phosphate cycle • • Stage 1: Fixation of CO2 into 3- Phosphoglycerae • 3- phosphoglycerate: the first compound labeled with 14C, predominantly located in the carboxyl carbon atom • C3 plants: the plants in which this three- carbon compound is the first intermediate • Rubisco, or ribulose 1,5- bisphosphate carboxylase/oxygenase: the enzyme that catalyzes the incorporation of CO2 into an organic form o Enzyme catalyzes the covalent attachment of CO2 to the five- carbon sugar ribulose 1,5- bisphosphate, which cleaves to form 2 molecules of 3- phosphoglycerate o Two forms of rubisco Form I is found in vascular plants, algae, and cyanobacteria • 8 subunits (Mr 53,000) • each contains a catalytic site and more smaller subunits Form II is simpler in structure o Inefficient, and low turnover number • Mg2+ ion brings together and orients the reactants at the active site and polarizes CO2, opening it to nucleophilic attack Rubisco Regulation • Rubisco remains inactive until carbamoylated on the epsilon amino group of Lys201 • Ribulose 1,5- bisphosphate inhibits carbamoylation by binding to the active site and lcoking the enzyme in the “closed” conformation • Rubisco activa...
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This note was uploaded on 02/06/2014 for the course BCHB 501 taught by Professor Jessicajones,elliottcrooke,ericgalsgow,marybethmartin during the Fall '12 term at Georgetown.

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