BIO130 Section One Guide (3)

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26 Apr 2011
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Chapter 3 Proteins
(pg. 125-152)
-Proteins such as kinesin propel organelles through the cytoplasm, topoisomerase
untangles knotted DNA molecules
The Shape and Structure of Proteins
The Shape of a Protein Is Specified by Its Amino Acid Sequence
-Proteins are long polymers of amino acids linked by peptide bonds, and they are
always written with the N-terminus toward the left
-* special case: two cysteine side chains can form disulfide bonds
Proteins Fold into a Conformation of Lowest Energy
-The final folded structure, or conformation , of any polypeptide chain is generally
the one that minimizes its free energy
-In living cells, special proteins called molecular chaperones often assist in protein
folding
oAlso prevent exposed hydrophobic regions in newly synthesized protein
chains from associating with each other to form protein aggregates
-Proteins are generally between 50-2000 amino acids long
oLarge proteins consist of protein domains which fold mostly independent of
each other
The α Helix and the Sheet are Common Folding Patterns
-α helix was found in the protein α-keratin, abundant in skin, hair, nails, horns
- sheet was found in the protein fibroin, the major constituent of silk
oboth patterns result from hydrogen bonding between N-H and C=O groups in
the polypeptide backbone
- sheets (forming the core of many proteins) can form either (via hydrogen bonds):
1. from neighbouring polypeptide chains running in the same orientation
(parallel)
2. from a polypeptide chain that folds back and forth upon itself (anti-
parallel chains)
-αhelix is made through twisting a single polypeptide chain around to form a rigid
cylinder
oA hydrogen bond is formed every 4th peptide bond (N-H and C=O)
oComplete turn every 3.6 amino acids
oFound in cell membranes (transport proteins, receptors)
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