Cryptochrome is a class of flavoprotein that acts as a blue light photoreceptor involved in circadian rhythms in plants and animals. It was first observed in plants in the 1800s but was not identified until the 1980s. Cryptochrome has a similar structure to photolyase but serves different functions. It contains a single molecule of FAD that absorbs blue light. In plants, cryptochrome mediates responses to blue light such as growth, seedling development, and leaf/stem expansion.

1. CRYPTOCHROME
Presented by :- Panchal Rita C .
CBO :- 501
M.Sc Sem :- 3
Department of life sciences ,
H.N.G.U., Patan

2. CONTENTS
• Introduction
• Structure
• History and Discovery
• Properties

3. INTRODUCTION
• Cryptochrome is a class of flavoprotein that en
compass a blue light driven reaction cycle.
• Photolyase and cryptochrome are functionally
different, but possess similar photoactive dom
ains.
• Cryptochrome is involved in the circadian clock
s of plants and animals, and the sensing of mag
netic fields in a number of species.

4. HISTORY AND DISCOVERY
• Although Charles Darwin first documen
ted plant responses to blue light in the
1800s, it was not until the 1980s that r
esearch began to identify the pigment r
esponsible.

5. • By 1995, it became clear that the produ
cts of the HY4 gene and its two human h
omologous did not exhibit photolyase a
ctivity and were instead a new class of b
lue light photoreceptor hypothesized to
be circadian photo pigments.
• In 1996 and 1998, cry homologous were
identified in drosophila and mice, resp
ectively.

6. STRUCTURE

7. • The structure of cryptochrome involves
a fold very similar to that of photolyase,
with a singal molecule of FAD.
• These proteins have variable lengths an
d surfaces on the c-terminal end,.
• The Ramachandran plot shows that the
secondary structure of the CRY1 protein
is primarily a right-handed alpha helix w
ith little to no steric overlap.

8. PROPERTIES
• Purification of recombinant flavoproteins suc
h as those in the photolyse / cryptochrome f
amily of blue photoreceptors often leads to l
oss of chromophores during purification and
chromophore oxidation.
• For this reason, flavin composition and redox
state in recombinant flavoproteins must be s
upplemented with supporting data to conclu
de the chromophore composition and redox
state of the flavin in vivo .

9. • Arabidopsis cryptochrome which are kn
own to function as photoreceptors, hav
e been purified with near-stoichiometric
catalytic flavin .
• The recombinant dCry photoreceptor, h
owever, contains less than 5% flavin , a
n apparent contradiction to its known p
hotoreceptive function.

10. MECHANISM

11. Autophosphorylating kinase activity o
f cryptochrome:
• Phosphorylation has been found to contri
bute to regulatory processes of many p
hotoactive enzymes.
• Phosphorylation activity of cryptochrome
s, however has not been extensively stud
ied and the current data are contradictory.

12. • The most heavily studied cryptochrome kinas
e activity is that of Arabidopsis Cry1 and Cry2;
both of which play important roles in light-sti
mulated photo morphogenic responses in pla
nts and have been purified with near- stoichi
ometric levels of flavin.

13. • AtCry1 has shown stoichiometric ATP bi
nding within theactive site of the enzym
e.
• Both autophosphorylation and ATP bind
ing were shown for Cry1 as well, howev
er flavin dependence was not tested sin
ce human cryptochromes have yet to be
purified with greater than trace amount
s of flavin.

14. ● Photophysical Cryptochrome Prope
rties:
●As mentioned previously, absorption tran
sients of excited flavin have been isolated i
n photolyase .
● In photolyase, this technology has prove
n the long predicted radical mechanism of p
hotoreactivation .

15. • Excited flavin absorption, in the presen
ce of T<>T substrate, was measured at 6
90 nm between 0.05 ns and 2.8 ns after
excitation to observe the decay rate of t
he excited flavin.
• At this high wavelength, the only absorb
ing species is FADH-.

16. ●However, at shorter wavelengths above 500
nm both FADH¯ and the neutral radical abso
rb.
● Therefore the difference between the FADH
¯ decay curve at 690 nm and those recorded
at either 625 nm or 510 nm can be attributed
to formation of FADH°.
● Since there is no known substrate for crypt
ochromes, identification of reaction intermed
iates is not possible at this time.

17. FUNCTION
• In plants , cryptochromes are photoreceptor
proteins which absorb and process blue light
for function such as growth , seedling develo
pment and leaf and stem expansion .
• When plants are exposed to blue light they e
xperience a reduction in flavin pigments . Thi
s reduction activates the cryptochromes and
thus allows for growth and seedling develop
ment .

19. REFERENCES
• WWW.biologydiscussion.net
• WWW.GOOGLE.Com