MAINCHAIN PART OF AMINO ACIDS

How do you join up α-helices and β-strands?
The need for β-turn and loop structures
β- turns and loops are the two
conformational extremes that
exist at the surface of globular
proteins to link together the
individual α-helices and β-strands
in the protein core. They are
hydrophilic.
Features:
(1) A β-turn involves a SHARP reversal in the direction of a
polypeptide chain over just four amino acid residues.
(2) β-turns are sterically demanding, so a Gly residue is often
located in the middle of a β-turn.
(3) CONTRAST β-turns with loop structures which are often
found at protein surfaces with as many as 10-20 amino acid
residues.
Supersecondary structures
(“supersecondary” refers to all the secondary structures inside the structure)
The number of ways in which α-helices and β-strands can be combined
to form a protein is limited. So a number of common “folds” occur.
There are three major classes of protein structures:
1. All α-helix proteins eg: “globin fold” of myoglobin and haemoglobin
2. All β-sheet proteins eg: “immunoglobulin fold” of antibodies
3. Mixed α-helix / β-sheet proteins eg: “TIM fold” with a closed β-sheet
barrel buried in the core and α-helices flanking this on the surface - or
the other “fold” variant based on an open twisted β-sheet buried in the
core with α-helices above and below it.
Globin Immunoglobulin Open twisted α/β TIM barrel
CONCLUSIONS
You have now met the 20 amino acids,
and seen how proteins are folded up.
(1) The 20 amino acids possess very different
chemical and physical properties.
(2) Proteins fold up using α-helices and β-sheets.
It is now down to you to read up
this topic in your textbook, and do the exercises.
Based on this lecture and the next, two Special
Study Modules will be offered on “Structure and
Function of Medically Important Proteins in
Disease” which is run jointly with a clinician.