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Quantum Effects and Light-harvesting:
From the algal cell’s point of view
Beverley R. Green
Botany Department, University of British Columbia,
Vancouver, B.C., Canada
RC2
RC1
RC2
RC1
FCPs
LHCI
RC2
CP26
CP29
RC1
LHCII
trimer LHCI
Light-harvesting from the algal cell’s point of view
The BIG QUESTIONS:
•How does it work?
•How did it get that way?
•Does it matter?
Three views of the cryptophyte alga Guillardia theta
SEM image
(G. McFadden)
Live cells
(phase contrast microscopy)
(Meriem Alami)
Fluorescence confocal image
showing “wrap-around” plastid
(Yunkun Dang)
Light-harvesting antennas use only 3 types of chromophore!
•
Chlorophylls and bacteriochlorophylls
• Absorb in blue and red/infrared regions.
• Phycobilins - open-chain tetrapyrroles
• Absorb 490-600 nm (green/yellow)
• Carotenoids –absorb and protect
• Harvest light but also convert energy from excited
chlorophyll to heat - photoprotection
1Chl*
Car
heat
(detected as fluorescence quenching (NPQ)
The proteins make the difference--> huge variety of antennas
B.R. Green (2001) “Was ‘molecular opportunism’ a factor in the evolution of different photosynthetic lightharvesting pigment systems?” PNAS 98: 2119-2121
Variety of Light-harvesting Antennas
Plant s
Cryptophyte algae
Dinoflagellate algae
Green sulfur bacteria
Purple bacteria
See “Light-harvesting Antennas in Photosynthesis” (2003) ed. B.R.Green, W.W. Parson.
Example #1: Purple bacterial LH2 antenna
Ostroumov et al. Science (2013) 340, 52–56.
Robert et al. (2003) Chap.4 in “Light-harvesting
Antennas in Photosynthesis”, B.R.Green &
W.W.Parson, Eds.
Example #2: Phycobilisome of cyanobacteria, red and glaucophyte algae
An energy cascade to RC2
PEF575nm > PCF640nm > APCF660nm > LCMF682nm >Chl (RC2)
phycoerythrin
phycocyanin
allophycocyanin
Chl
Photosynthetic
membrane
PE
PC
APC
Absorbance
phycobilisome
reaction centre
linker
RC2
Chll
RC1
0
400
450
500
550
600
650
700
Wavelength (nm)
Phycobilisome doesn’t bind carotenoids.
But its efficiency is regulated by an orange carotenoid protein.
Example #3: The LHC Superfamily:
•Eukaryotic invention
•No energy cascade
•Pigments hung all over the place
•Big protein family
•Carotenoids -both for photoprotection
and light-harvesting
Plant LHCII
Liu et al. (2004).
RC2
RC2
RC1
RC2
Lhcs
Lhcs
Green algae and plants -Chl a/b
RC1
RC1
“Brown” algae -Chl a/c
Lhcs
Red algae-Chl a only
Cryptophytes-an amazing evolutionary story!
Chloroplasts acquired by secondary endosymbiosis
Chloroplast envelope
(2 membranes)
N2
N2
N1
2nd Host
(heterotroph) Red alga
Phycobilisome
Nucleomorph
Cryptophyte
Gene loss
Photosynthetic
membrane
RC2
Red alga
RC2
RC1
Gene gain:
new α subunit
RC1
LHC
<-- NEW!!
Cryptophyte

A brand new antenna from parts:
β subunit from ancestral phycobilisome
α subunit from…?
Assembled tetramer relocated to thylakoid lumen
2
NEW α’s
1

Anatomy of a cryptophyte phycobiliprotein
α1 subunit (red)
9 kDa
β subunit
19 kDa
β subunit
19 kDa
α2 subunit (blue)
8 kDa
oAlpha subunits are the key: interactions between the α subunits and
between α and β subunits hold the tetramer together.
oThey are the most variable in sequence.
From Macpherson and Hiller (2003) in Light-harvesting Antennas in Photosynthesis (ed. B.R.Green, W.W. Parson).
Based on x-ray structure of Wilk et al, PNAS 96: 8901-8906, 1999.
Cryptophytes also evolved a variety of new phycobilin pigments
α subunit binds 1 phycobilin
β subunit binds 3 phycobilins
Result: great variety in wavelengths of light absorbed.
PC 645 demonstrates electronic coherence
PC645 from Chroomonas sp.
Light-harvesting from the algal cell’s point of view
PC 645 from Chroomonas demonstrates electronic coherence
So does PE545 from Rhodomonas
Pair of DVB
The two central bilins are in van der Waal’s contact and strongly coupled
Light-harvesting from the algal cell’s point of view
X-ray Crystallography produced a surprise:
Hemiselmis phycobiliproteins are different from all the others.
(H. andersenii: PE555, H. virescens: PC612, H. pacifica: PC577)
“Open”
Hemiselmis-PC612, PE577, PE555
“Closed”
Chroomonas-PC645
and all the others
Hemiselmis-the two αβ units are rotated 73º with respect to each other, giving a hole down the
middle (“open” configuration) and the central pair of pigments are no longer in contact.
Harrop et al. (2014) Single-residue insertion switches the quaternary structure and exciton states of cryptophyte
light-harvesting proteins. PNAS Early Edition: www.pnas.org/cgi/doi/10.1073/pnas.1402538111
A single amino acid insertion (- charge) in the α subunit sequence
is responsible for the open conformation!
Aspartic acid that
forces the Open
conformation
-Sheet S1
Hemiselmis (open)
HemiPC557
HemiPC612
HemiPE555
-Sheet S2
-helix
KMAKDSKAPVVEIFDERDGCTSAGST~~~~GKASDAGEKGLLVKVSMQKVGYNAIMAKSVAASYMNK
KMATDSKAPLIELFDERDGCKGPAAN~~~~~KASDVGEPGLCVKVSMQKVAMNAAAAKSVATNYMRK
AMKKDSKAPCVEVFDERDGCKAAGTQ~~~~~KAS~~GDDGFCVKVSMKAIGFNAAEAASVTKNYGIKRFGAKSV
Chroomonas(closed)
C1627PC630
AIKKDQKAPVVTIFDAR~GCKDHSNKEYTGAKAGGM~EDDQCVKLTMETIKVGDDVAAKVLGECLSELKSRK
C1312PC630
AIKKDQKAPVITIFDAR~GCKDHANKEYTGAKAGGM~DDEQCVKLTMETIKVADDVAASVLREALGELKSK
CmesPC645
KDAQLRAPVVTIFDAR~GCKDHANKEYTGPKAGNAENDECCVKVQMTPIKVADDAAALVLKECLSELKGKK
ChrPC645
KDAQLRAPVVTIFDAR~GCKDHANKEYTDPKAGNAENDECCVKVQMTPIKVADDAAALVLKECLSELKG
•Found only in Hemiselmis species ..(“open” form)
•Causes a chain-reaction of rearrangements of amino acids….the two αβ units cannot fit
closely together.
•This mutation appears to have arisen within the Chroomonas clade and been amplified by
gene duplication.
Strong excitonic coupling in closed form only
Closed structures
Open structures
Harrop et al. (2014) Single-residue insertion switches the quaternary structure and exciton states of cryptophyte
light-harvesting proteins. PNAS Early Edition: www.pnas.org/cgi/doi/10.1073/pnas.1402538111.
2D Electronic Spectroscopy shows electronic and vibrational coherences in closed form
Closed
Open
Open
Harrop et al. (2014) Single-residue insertion switches the quaternary structure and exciton states of
cryptophyte light-harvesting proteins. PNAS Early Edition: www.pnas.org/cgi/doi/10.1073/pnas.1402538111
Another twist to the story: “BIG DATA” changed the picture!
Marine Microbial Eukaryote Transcriptome Project
John Archibald and Naoko
Tanifugi, Dalhousie University,
Halifax
Hemiselmis species have genes for both “open” and “closed” α subunits!
Some of the “closed” form genes were as highly expressed as the “open” forms!
Cell used for mRNA isolation and sequencing were grown under
HIGH LIGHT (100-150 mol m-2 sec-1)
Cells used for our targeted gene sequencing, crystallography and spectroscopy were
grown under
LOW LIGHT! (20-40 mol m-2 sec-1)
This suggests that electronic coherence may play a part in ACCLIMATION to high
light intensity by allowing synthesis of “closed” forms, ie. quantum switching.
An elegant test:
Will the "closed" form purified from an "open" species demonstrate electronic coherence?
This test of the relationship between protein conformation and coherent behavior awaits funding.
The Cryptophyte Phycobiliprotein Consortium:
Molecular biology and sequence analysis
(Vancouver and Sydney, Australia)
•Beverley Green
•Chang Ying (Ivy) Teng
•Roger Hiller
Crystallography (Sydney, Australia)
•Paul Curmi
•Stephen Harrop
•Krystyna Wilk
•Roger Hiller
•
Spectroscopy (Toronto)
•Gregory Scholes
•Tihana Mirkovic
•Daniel Turner
•Rayomond Dinshaw
•Daniel Oblinsky
Genomics/transcriptomics (Halifax)
•John Archibald
•Naoko Tanifugi
Dr. Kerstin Hoef-Emden (Köln) published a key paper relating cryptophyte evolution and
phycobiliproteins (J. Phycology 44: 985-993, 2008), and provided cultures and much advice.
We gratefully thank DARPA for funding research on cryptophyte photosynthesis!
•Thanks also to the Australian Research Council, the Natural Sciences and Engineering Research Council of
Canada, the Moore Foundation and DOE-JGI Eukaryotic Genomics.
Biologists do it differently…..
•High-throughput (aka brute force) methods, e.g. genome sequencing.
•Mutant isolation—mutate first, ask questions later.
•Relative (comparative) measures to understand an effect. Internal controls.
•Bottom up (experiment to theory) as opposed to top down (theory to experiment)
Aureococcus
Anophagefferens
(brown tides)
Thalassiosira
pseudonana
(diatom)
Phaeodactylum
tricornutum
(diatom)
Fragilariopsis
cylindrus
(diatom)
Emiliania
Huxleyii
(haptophyte)
Guillardia theta
(cryptophyte)