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Protein Structure and Function
- Proteins catalyze reactions, bind to molecules and
transport molecules
- Proteins can do these diverse functions because of
their complex shapes and the precise positioning of
various chemical groups.
- The twenty
amino
acids have a variety of functional groups: acidic,
basic, hydrophobic, hydrophilic, cyclic, large, small,
etc.
- Four levels of protein structure - primary,
secondary, tertiary and quaternary
- Primary - the linear order of the amino acids in
the polypeptide chain
- The amino group of one amino acid reacts with
the carboxyl group of the next amino acid to form
the peptide bond
- There is a free amino group at one end (where
synthesis starts) and a free carboxyl group at the
other end (where synthesis ends)
A more extensive tutorial on amino acid and peptide
structure can be found at the
Amino
Acid tutorial, however, you will need the
Chime
plugin to view this tutorial.
- Secondary structure refers to stable structures
formed in regions of the polypeptide chain. The most
important are the alpha helix and the beta-pleated sheet
- There are also turns, random coils, etc.
- The alpha helix is stabilized by hydrogen bonds
- bonds go length ways, not across as in DNA
- 3.6 amino acids per turn
- side groups stick out, not in as in DNA
- Amphipathic helix has hydrophobic groups on one
side, hydrophilic on the other
For those with the chime plugin there is a much more
extensive example of
protein
strucutre at Carnigie Mellon University
- Beta-pleated Sheets
- Two stretches of the polypeptide chain lie
parallel to one another and are held together by
hydrogen bonds
- can be parallel or antiparallel
- multiple chains held together this way create a
sheet
- sheets can be very strong and inflexible
- Tertiary Structure - the three dimensional
conformation of the entire chain
- Can be measured using NMR or X-Ray crystallography
- Held together by covalent disulfide bonds between
cysteines, hydrogen bonds in alpha helix and
beta-pleated sheets, and the tendency of hydrophilic
groups to face the water and hydrophilic groups to
clump together in the interior
- Many proteins need chaperones to help them
fold properly
Beta subunit of hemoglobin with heme co-factor in red
or, with the alpha helixes highlited
- Quaternary Structure
- Many proteins are composed of multiple polypeptide
chains, some also include co-factors
- hemoglobin is a tetramer of two _ chains and two _
chains
- each chain also includes a heme group
- cooperative binding of oxygen is very important
for proper hemoglobin function
For the chime enabled there is much more at this
hemoglobin
site
This document is copyright of
Jeff
Bell
Last Update: Wednesday, March 28, 2001
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