Alpha Helix and Beta Sheets/Turns

Alpha Helix and Beta Sheets/Turns

1. Alpha Helix: SAS

   • SIMPLE AND STABLE
     • NH-down; CO-up; R-out
     • Mostly Right handed
       • L AMINO ACIDS
     • Low propensity
       • GLY
         • low steric constraint; go anywhere
       • PRO
         • RIGID; breaks helices

2. Beta Sheets: PLEATED

   • Antiparallel
     • TWO AA H-bond per group
       • Aligned
         • STRONG
   • Parallel
     • H-Bond at ANGLES
       • BENT
         • WEAK
   • Beta barrel
     • Parallel sheets in cylinder
       • Tirose Phosphate isomerase
   • Silk
     • Strong Beta sheet
       • GLY, SER (alternate)
   • Propensity
     • HIGH
       • NONPOLAR
     • LOW
       • NONPOLAR
       • PRO
       • GLY
   • BURIED

3. Beta turns

   • When a protein chain makes a 180 turn
     • EXPOSED
       • H-bond exists from C=O group of #i to N-H on #i+3
         • Propensity
           • HIGH
             • POLAR
             • PRO
               • #i+1
             • GLY
               • Frequently observed b/c:
                 • Formation of H-bonds
                 • Steric considerations
                 • Polar v. non-polar characteristics
           • LOW
             • NONPOLAR