HELIX-LOOP-HELIX, HELIX-TURN-HELIX

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 bind DNA   ( 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 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 19

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

21. THANK YOU 21