Protein Structure
beta sheets are twisted
• Parallel sheets are less twisted than antiparallel and are always buried.
The twist is due to chiral (l)- amino acids in the
extended plane.
This chirality gives the twist and distorts
H-bonding.
Two proteins exhibiting a twisting
sheet
Bovine carboxypeptidase
Sheet facts
•Repeat distance is 7.0 Å
•R group on the Amino acids alternate up-down-up above and below the plane of the sheet
•2 - 15 amino acids residues long
•2 - 15 strands per sheet
•Ave of 6 strands with a width of 25 Å
•parallel less stable than anti-parallel
•Anti-parallel needs a hairpin turn
Non-repetitive regions
Turns - coils or loops link regions of secondary structure 50% of structure of globular proteins are not
repeating structures
bends
Folding motifs (super secondary
structure)
Certain amino sequences have patterns to their folding.
A. motif, B. hairpin C. motif
beta-alpha-beta
• parallel beta-strands connected by longer regions containing alpha-helical segments
Helix-turn-helix
• loop regions connecting alpha-helical segments can have important functions e.g. EF-hand and DNA-binding
• EF hand loop ~ 12 residues
• polar and hydrophobic a.a. conserved positions • Glycine is invariant at the sixth position
Protein Folds
There is an estimate of about 10000 different folding
patterns in proteins
About half of the proteins fall into a few dozen folding
patterns.
The barrel has several types of
structures that have been mimicked in art.
A. rubredoxin
B. Human prealbumin or porins
Concanavalin A
Carbonic anhydrase
H
2CO
3 -
CO
Glyceraldehyde-3-phosphate
dehydrogenase
Zinc fingers
C2H2 zinc finger: It is characterized by the sequence CX2-4C....HX2-4H,
where C = cysteine, H = histidine, X = any amino acid.
C4 zinc finger: Its consensus sequence is CX2CX13CX2CX
14-15CX5CX9CX2C. The first four cysteine residues bind to a zinc ion and
the last four cysteine residues bind to another zinc ion
C6 zinc finger. It has the consensus sequence CX2CX6CX5-6CX2CX6C.
Summary Chapter 6
• Four levels of protein structure
– Primary
