Chapter 20 Biochemistry Key Terms

calvin cycle/photosynthetic carbon reduction cycle
The cyclic pathway in plants
that fixes carbon dioxide and produces triose
In plants, a self-replicating organelle bound by double membrane and contains a small genome that encodes some of their proteins;
may differentiate into a chloroplast
Chlorophyll-containing photosynthetic
organelles in some eukaryotic cells; sites of CO2 assimilation; enzymes for this process in stroma; green leaves are reach in these
colorless plastids (lack chlorophyll and other pigments found in chloroplasts); no internal membrane analogous to photosynthetic membranes (thylakoids) of chloroplasts; in plant tissue rich in starch, are packed w/ starch granules; dominate in nonphotosynthetic tissues that store starch in large quantities
carbon fixation rxn/co2 fixation
The reaction,
catalyzed by rubisco during photosynthesls or
by other carboxylases, in which atmospheric
CO2 is initially incorporated (fixed) into a three-C
organic compound, the triose phosphate 3 phosoglycerate; condensation of CO2 w/ five-carbon acceptor, ribulose 1,5 bisphosphate to form 2 molecules 3-phoshoplycerate
pentose phosphate pathway
A pathway
present in most organisms that serves to
interconvert hexoses and pentoses and is a
source of reducing equivalents (NADPH) and
pentoses for biosynthetic processes; it beglns
with glucose 6-phosphate and includes 6-
phosphogluconate as an intermediate. Also
called the phosphogluconate pathway and the
hexose monophosphate pathway
reductive pentose phosphate cycle
photosynthetic assimliation of CO2, essentially same set of rxns as nonoxidative phase of pentose phosphatte pathwway operating in other direction, going from hexose phosphate to pentose phosphates; uses same enzymes and several more enzymes that make reductive cycle irreversible; all 13 enzymes are in chloroplast stroma
c3 plants
3 phosphoglycerate as first intermediate
ribulose 1,5 bisphosphate carboxylase/oxygenase (rubisco)
enzyme that catalyzes incorporation of C02 into organic form (3-phosphoglycerate)
rubisco activase
overcomes inhibitions of carbamoylation by binding tightly active site and locking the enzyme "closed" conformation by promoting ATP-dependent release of ribulose 1,5 bisphosphate, exposing Lys amino group to nonenzymatic carbamolyation by CO2, followed by Mg2+ binding, activating the rubisco.
enzyme used in stage 3 of Carbon assimilation; in step one, first catalyzes the reversible condensation of glyceraldehyde 3-phosphate w/ dihydroxyacetone phosphate, giving fructure 1,6-biphosphate; acts again in step 4 combining erythrose 4-phosphate w/ dihydroxyacetone phosphate to form 7-carbon sedoheptulose 1,7-bisphosphate
enzyme used in stage 3 of Carbon assimilation; has TPP as prosthetic group and requires Mg2+. step 3, catalyzes the reversible transfer of 2-C ketol group from a ketose phosphate donor, fru-6-phosphate, to an aldose phosphate acceptor, gly-3-phos, forming pentose xylulose 5-phosphate and tetrose erythrose 4-phos.; in step 6, converts sedoheptulose 7-phos and gly-3-phos to two pentose phosphate.
carbon-assimilation rxns
Reaction sequences in which atmospheric CO2 is
converted into organic compounds
with illumination, electrons flow from PSI to ferredoxin which passes electrons to a small, soluble, disulfide-containing protein called this
ferredoxin-thioredoxin reductase
catalyzes thioredoxin
photorespiration/oxidative photosynthetic carbon cycle (c2 cycle)
Oxygen consumption occurnng in illuminated temperate-zone plants, largely due to oxidation of
phosphoglycolate; consume O2 and produces CO2, and like photosynthesis, driven by light. costly side rxn of photosynthesis.
formed when rubisco catalyzes the condensation of O2 w/ ribulose 1,5-bisphosphate, and as a result forms 3-phosphoglycerate and this
glycolate pathway
converts two molecules of 2-phosphoglycolate to a molecule of serine and a molecule of CO2
c4 plants
phosphoenolpyruvate carboxylase
when first intermediate into which 14 CO2 is fixed is oxaloacetate, a four-carbon compound, this occurs in cytosol and is catalyzed by this; for c4 plants
malic enzyme
in c4 plants, in bundle sheath cells, malate is oxidized and decarboxyate to yield pyruvate and CO2 by the action of this enzyme
pyruvate phosphate dikinase
The pyruvate formed by decarboxylation of malate
in bundle-sheath cells is transferred back to the mesophyll cells, where it is converted to PEP by an unusual enzymatic rxn catalyzed by this enzyme
CAM plants
see pg. 790
nucleotide sugars
starch synthase
sucrose 6-phosphate synthase
fructose 2,6 biphosphate
ADP-glucose pyrophosphorylase
cellulose synthase
A major component of bacterial
cell walls; generally consists of parallel
heteropolysaccharides crosslinked by short
metabolite pools
co2 assimilation
green plants contain chloroplasts unique enzymatic machinery that catalyze conversion of CO2 to simple (reduced) organic compounds
converted from chloroplasts that lose internal membranes and chlorophyll and are interconvertible w/ amyloplasts
ribulose 1,5 bisphosphate
five-carbon acceptor that condenses w/ CO2 to form 2 molecules 3-phosphoglycerate
forms from condensation of CO2 w/ ribulose 1,5 bisphosphate; dominant w/ 14C in carboxyl carbon atom, early intermediate in photosynthetic
3-phosphoglycerate kinase
in first step of stage 2 of C assimiliation, this catalyzes the transfer of phosphoryl group from ATP to 3-phosphoglycerate, giving 1,3-biphosphoglycerate
glyceraldehyde 3-phosphate dehydrogenase
enzyme in stage 2 of C assimilation in which NADPH donates electrons in a reduction catalyzed by this chloroplast-specific isozyme, giving glyceraldehyde 3-phosphate and Pi
sedoheptulose 1,7 bisphosphate
step 4 of stage 3 of C-assimilation, combining erythrose 4-phosphate w/ dihydroxyacetone phosphate to form 7-carbon sedoheptulose 1,7-bisphosphate
ribulose 5-phosphate
pentose phosphates formed in the transketolase
reactions-ribose 5-phosphate and xylulose bphosphate-are converted to this
mitochondrial respiration
oxidation of substrates to CO2 and conversion of O2 to H2O
glycine decarboxylase complex
oxidizes glycine to CO2 and NH3, w/ concomitant reduction of NAD+ to NADH and transfer of remaining carbon from glycine to cofactor tetrahydrofoqlate
c4 plants
temporary fixation of CO2 into a four carbon compound, step prior to CO2 ix fixed into a three carbon product, 3-phosphoglycerate
c4 metabolism/c4 pathway
c4 plants assimilation process
c3 plants
first step rxn of CO2 w/ ribulose 1,5 biphosphate to form 3-phosphoglycerate are called this types of plants