Light dependent carbon metabolism

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Light dependent carbon metabolism
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Light harvesting and light regulation we have
discussed to date.
Theoretically, rates of O2 evolution should equal
rates of CO2 uptake
O2 evolution/ CO2 uptake ratio varies from 0.8 to
1.2 in nature
Therefore, quantum yield estimates base upon Ps
II activity may not equal those based upon CO2
fixation. See board
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Rubisco is also known by older name, RUDP
carboxylase
Rubisco catalyzes the only known reaction by
which living organisms accrue carbon from
atmospheric CO2
Rubisco predates photosynthesis (and O2) and is
also present in chemoautotrophs
Rubisco is different in construct in prokaryotes
vs eukaryotes
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Rubisco one of most abundant protein (soluble
enzyme) in nature
8 large subunits (blue/ violet) highly conserved,
occurring as 4 dimers Codes on chloroplast DNA
one catalytic site per dimer
8 small subunits, (gold) required for function
of large subunits Coded in nucleus as multigene
family different genes expressed in response
to environmental changes, including light. Also
regulated by biological clock, giving it a
lightdark periodicity in abundance and activity.
Top View
Side View
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If CO2 binds to active site, then photosynthesis
with net increase in organic carbon
If O2 binds to active site, then
photo-respiration occurs with no gain in organic
carbon and possible loss if glycolate,
accumulated in peroxisomes, is excreted to
environment rather than recycled..
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O2 can also originate within Ps II
Reason O2 content during Experiments are so
critical.
Purpose unknown.. Speculate??
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Rubisco Localized in stroma, may accumulate in
pyrenoid in some algae
Rubisco in algae with pyrenoids have higher Ks
than those who have Rubisco in stroma only.
Rubisco is considered a very inefficient enzyme
due to
1. Competing carboxylase/oxygenase
2. Very slow CO2 fixation rate (3 sec!)
3. Must regenerate substrate for CO2 fixation
before second CO2 molecule can be fixed.
Rubisco regulation is very complex and not fully
understood. e.g. Rubisco activase.
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Why is Rubisco fixation of CO2 more challenging
in SW than FW??
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CA Carbonic anhydrase
Carbonic anhydrases catalyze the inter-conversion
of carbonic acid and carbon dioxide HCO3- H
-----gt CO2 H2O
among the fastest enzymes known, with turnover
numbers close to one million per sec.
Depending upon type of CA, located in different
membranes of the cell
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Rubisco can exist in an inactive state due to
phosphorylated inhibition of active site where
CO2 or O2 .
Rubisco activase is required to acitvate Rubisco
(by removing phosphorylation)
Why have such a regulatory system.. Is it
important that Rubisco be inactivated when it
accumulates in pyrenoids or stroma sites??
Highly complex regulation of Rubisco.
Determining concentration does not mean
determining activity.
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(1) one or more chloroplast membrane-located Ci
transport systems that use photosynthetic energy
to accumulate HCO3 within the chloroplast
x
x
x
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(2) pyrenoid or stromal Rubisco, which may be
an integral part of CCM activity
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x
x
x
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Speculative based upon unexplainable observations
that dont work with (1) or (2)
(3) a pyrenoid carbonic anhydrase (CA), which is
hypothesized to be needed on kinetic grounds, but
as yet there is no concrete evidence for its
existence (see Badger and Price 1992)
(4) a thylakoid CA that might use lumenal protons
to generate CO2 in either the pyrenoid or the
stroma surrounding the thylakoids (see Raven
1997) however, there is little
experimental evidence for the existence of this
mechanism
(5) an envelope CA that could minimize CO2
leakage by converting CO2 back to HCO3 ñ this is
entirely speculation but has been suggested (see
Raven 1997).
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How might the inefficiencies of Rubisco be
overcome??
Range of CO2 concentrating mechanisms.. Not in
all phytoplankton, sometime more than 1 kind
possible in same phytoplankton
CA carbonic anhydrase.
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Photosynthesis measured as O2 evolution (PS II)
or CO2 uptake (fixation)
Theoretically, the rate of O2 evolution should
equal rate of CO2 uptake.
Review of P-I parameters
Pmax light-saturated photosynthetic capacity.
Pmax/cell is important to know potential for
growth, Pmax/Chl for physiological state.
Alpha light limited rates of Ps. Alpha/Chl is
indicator of relative quantum yield
Ik irradiance required for onset of
light-saturated Ps Pmax/alpha