ALL BASIC AND IMPORTANT CONTENT REGARDING HELIX LOOP HELIX AND HELIX TURN HELIX.. ALL ITS FUNCTION AND SUMMARY. HOPE YOU WILL FIND IT EASY..

1. HELIX- TURN- HELIX
&
HELIX- LOOP- HELIX
BIOTECHNOLOGY
- BY- NAREN
YADAV

2. DNA – Binding Domain
A DNA-binding domain (DBD) is an
independently folded protein domain that
contains at least one motif that recognizes
double- or single-stranded DNA. A DBD can
recognize a specific DNA sequence
(a recognition sequence) or have a general
affinity to DNA.
2

3. Types of DNA-Binding Motifs
• 1. Steroid Receptor
• 2. Zinc Fingers
• 3. Leucine Zipper
• 4. Homeodomain
• 5.HELIX-TURN-HELIX
• 6.HELIX-LOOP-HELIX
3

4. Helix-turn-helix
• In proteins, the helix-turn-helix (HTH) is a
major structural motif capable of
binding DNA.
4

5. DISCOVERY
• The discovery of the helix-turn-helix
motif was based on similarities
between several genes
encoding transcription regulatory
proteins from bacteriophage
lambda and Escherichia coli: Cro, CAP,
and λ repressor, which were found to
share a common 20-25 amino
acid sequence that facilitates DNA
recognition. 5

6. EXAMPLE
6
The λ repressor of
bacteriophage
lambda employs a
helix-turn-helix (left;
green) to
bindDNA (right; blue
and red).

7. STRUCTURE
• The structure consist of two α helices joined
by a short strand of amino acid.
• The recognition and binding to DNA by
helix-turn-helix proteins is done by the
two α helices, one occupying the N-
terminal end of the motif, the other at
the C-terminus.
• In most cases, such as in the Cro
repressor, the second helix contributes
most to DNA recognition, and hence it is
often called the "recognition helix".
7

8. Helix-turn-helix
C-terminal binds to major groove, N-terminal
helps to position the complex
8

9. FUNCTION
 The helix-turn-helix (HTH) is a major structural
motif capable of binding DNA. It is found in
many proteins that regulate gene expression.
 It binds to the major groove of DNA through
a series of hydrogen bonds and various Van
der Waals interactions with exposed bases.
The other α helix stabilizes the interaction
between protein and DNA, but does not play
a particularly strong role in its recognition.
 The recognition helix and its preceding helix
always have the same relative orientation.
9

10. 10

11. CLASSIFICATION OF HELIX
TURN HELIX
Di-helical
Tri-helical
Tetra-helical
Winged helix-turn-helix
Other modified helix-turn-helix motifs
11

12. Helix-turn-helix
review
LOCATION
•Bacterial
regulatory
proteins,
•Related motifs
in eukaryotic
proteins.
CHARACTERIST
-ICS
•Two alpha
helices
BINDING SITE
IN DNA
•Major groove
12

13. Helix-loop-helix
- The helix-loop-helix binding motif
consists of two alpha helices separated
by a loop of amino acids.
13

14. STRUCTURE
-Two polypeptide chains with the motif
join to form a functional DNA-binding
protein.
- A highly basic set of amino acids in one
of the helices binds to the DNA.
- Transcription factor including this
domain are in dimeric form.
14

15. CONTINUE…
• In general, one helix is smaller, and,
due to the flexibility of the loop, allows
dimerization by folding and packing
against another helix.
• The larger helix typically contains the
DNA-binding regions.
• bHLH proteins typically bind to
a consensus sequence called an E-box,
CANNTG.
15

16. FUNCTION
16
 bHLH transcription factors are often
important in development or cell
activity. BMAL1-Clock is a core
transcription complex in the
molecular circadian clock.
 Other genes, like c-Myc and HIF-1,
have been linked to cancer due to
their effects on cell growth and
metabolism.

17. REGULATION
• Since many bHLH transcription
factors are heterodimeric, their activity
is often highly regulated by the
dimerization of the subunits.
• One subunit's expression or availability
is often controlled, whereas the other
subunit is constitutively expressed.
17

18. 18

19. Helix-loop-helix
review
LOCATION
•Eukaryotic
protein
CHARACTERIST
-ICS
•Two alpha
helices
separated by a
loop of amino
acids.
BINDING SITE IN
DNA
•Major groove
19

20. EXAMPLES
• AhR,BMAL-1-CLOCK,C-Myc,
• N-Myc,MyoD,Myf5,Scleraxis,
• Neurogenis, etc…
20

21. •THANK YOU
21