Thanks to, Sri Parama kalyani college, Alwarkurichi

1. SRI PARAMAKALYANI COLLEGE,
Reacredited with B grade with a CGPA of 2.71 in the second cycle of NAAC
Affiliated to Manonmaniyam Sundaranar University, Tirunelveli.
ALWARKURICHI-627412.
Post graduate & Research Centre – Department of Microbiology
(government aided)
1st SEM CORE; MICROBIAL PHYSIOLOGY AND METABOLISM (ZMBM13)
UNIT-4
BACTERIAL RHODOPSIN
Submitted by,
VASIMA .A
REG. NO: 20211232516125
1st M.SC MICROBILOGY
Submitted to,
GUIDE: Dr. S. VISHWANATHAN,PH. D.,
ASSISTANT PROFESSOR & HEAD,
SPKC – ALWARKURICHI.

2. SYNOPSIS:
• INTODUCTION
• STRUCTURE OF BACTERIAL RHIDOPSIN
• PROPERTIES
• FUCNTION
• MECHANISM
• REFERENCE

3. INTRODUCTION:
• Bacteriorhodopsin is a protein used by Archaea, most
notably by haloarchaea, a class of the Euryarchaeota.
• It acts as a proton pump; that is, it captures light energy and
uses it to move protons across the membrane out of the cell.
• The resulting proton gradient is subsequently converted into
chemical energy.

4. STRUCTURE:
• Bacteriorhodopsin is a 27 kDa integral membrane
protein usually found in two-dimensional crystalline patches
known as "purple membrane", which can occupy almost 50%
of the surface area of the archaeal cell.
• The repeating element of the hexagonal lattice is composed
of three identical protein chains, each rotated by 120 degrees
relative to the others.
• Each monomer has seven transmembrane alpha helices and
an extracellular-facing, two-stranded beta sheet.

5. STRUCTURE OF RHODOPSIN
The structure of bacteriorhodopsin
consists for the most part of seven
transmembrane helices. This seven-
helix tertiary structure is a common
protein fold for integral membrane
proteins.

6. PROPERTIES:
• Bacteriorhodopsin molecule is purple and is most efficient
at absorbing green light (in the wavelength range 500-
650 nm).
• In the native membrane, the protein has a maximum
absorbance at 553 nm, however addition of detergent
disrupts the trimeric form, leading a loss of exciton coupling
between the chromophores, and the monomeric form
consequently has an absorption maximum of 568 nm.

7. CONTI…
• Bacteriorhodopsin has a broad excitation spectrum. For a
detection wavelength between 700 and 800 nm, it has an
appreciable detected emission for excitation wavelengths
between 470 nm and 650 nm (with a peak at 570 nm).
• When pumped at 633 nm, the emission spectrum has
appreciable intensity between 650 nm and 850 nm.

8. FUNCTION:
• Bacteriorhodopsin is a light-driven H+ ion transporter
found in some haloarchaea, most notably Halobacterium
salinarum (formerly known as syn. H. halobium).
• The proton-motive force generated by the protein is used
by ATP synthase to generate adenosine triphosphate
(ATP).
• By expressing bacteriorhodopsin, the archaea cells are able
to synthesise ATP in the absence of a carbon source.

9. CONTI…
Chemiosmotic coupling between
bacteriorhodopsin and ATP
synthase in the Halobacterium
salinarum membrane

10. MECHANISM:
• Bacteriorhodopsin is a light-driven proton pump. It is the retinal
molecule that changes its isomerization state from all-trans to
13-cis when it absorbs a photon.
• The surrounding protein responds to the change in the
chromophore shape, by undergoing an ordered sequence
of conformational changes (collectively known as the
photocycle).
• The conformational changes alter the pKa values of conserved
amino acids in the core of the protein, including Asp85, Asp96
and the Schiff base N atom (Lys216).

11. CONTI…
• These sequential changes in acid dissociation constant, result in the
transfer of one proton from the intracellular side to the extracellular
side of the membrane for each photon absorbed by the
chromophore.
• The bacteriorhodopsin photocycle consists of nine distinct stages,
starting from the ground or resting state, which is denoted 'bR'.

12. CONTI…

13. CONTI…
• The intermediates are identified by single letters and may
be distinguished by their absorption spectra. The nine
stages are:
• bR + photon → K ⇌ L ⇌ M1 ⇌ M2 ⇌ M2' ⇌ N ⇌ N' ⇌
O ⇌ bR

14. MECHANISM CONTI…
• Ground state + photon → K state → L state: Bacteriorhodopsin in
the ground state absorbs a photon and the retinal changes
isomerization from all-trans 15-anti to the strained 13-cis 15-anti in
the K state. The isomerisation reaction is fast and occurs in less than 1
ps. The retinal adopts a less strained conformation to form the L
intermediate.
• L state → M1 statet: Asp85 accepts a proton from the Schiff base N
atom. In the M1 intermediate, neither the Schiff base nor Asp85 are
charged.

15. CONTI…
• M1 state → M2 state: The Schiff base rotates away from the
extracelluar side of the protein towards the cytoplasmic side, in
preparation to accept a new proton.
• M2 state → M2' state: A proton is released from Glu204 and Glu194 to
the extracellular medium.
• M2' state → N state: The retinal Schiff base accepts a proton from
Asp96. In the N state, both Asp96 and the Schiff base are charged.
• N state → N' state: Asp96 accepts a proton from the cytoplasmic side
of the membrane and becomes uncharged.

16. CONTI…
• N' state → O state: Retinal reisomerizes to the all-trans state.
• O state → ground state: Asp85 transfers a proton to Glu195 and
Glu205 on the extracellular face of the protein.

17. BACTERIAL RHODOPSIN
Purple bacteriorhodopsin in H
alobacteria at Cargill's salt
evaporation ponds in San
Francisco Bay, located
at Newark, California

18. REFERENCE:
• Bacteriorhodopsin: Molecule of the Month, by David
Goodsell, RCSB Protein Data Bank.
• Blanck A, Oesterhelt D, Ferrando E, Schegk ES, Lottspeich
F (December 1989). "Primary structure of sensory rhodopsin
I, a prokaryotic photoreceptor.