Biochemistry Lecture No. 6: Protein Complexes:
Tuesday September 18 , 2012
-β sheets can be formed from two or more separate polypeptide chains, OR a single polypeptide folding
back on itself.
-pK = pH – log ([RCOO] / [RCOOH]) pK = - log K
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-Complexes are known for containing many proteins and all are required to perform some job within the
cell. Each protein must recognize and bind to others in the complex, thus assembling into a
supramolecular structure. Two classic examples of important protein complexes are the ribosome and
the proteasome. The ribosome, involved in protein synthesis, is a specific complex of more than 50
protein and RNA molecules. The proteasome, involved in protein degradation, is a specific complex of
about 20 proteins.
- With a molecular weight of 700,000, the proteasome is an enormous protein complex. It uses ATP to
unfold proteins and later recycle the resultant amino acids. Best visualized as a “barrel-shape,” proteins
for degradation enter the proteasome and exit as single amino acids to be recycled.
The Ribosomal Complex:
-Composed of two large subunits, the ribosomal complex houses an array of protein and RNA molecules
as both alpha helical structures (shown as a ribbon structure) for the process of translation.
-The most common protein in animals (about 25%), collagen connects and strengthens tissue.
Extracellular in nature, it forms strong fibres or sheets throughout the bone matrix, tendons, connective
tissue, and skin, making it essential to proper human function. Collagen belongs to the family of proteins
numbered I – XIV and is known for its characteristic triple helical structure appearance.
-Many proponents of commercial anti-aging supplements and cosmetics containing collagen have very
little scientific evidence to claim the products work as collagen like all other dietary proteins will
degrade in the body as soon as it reaches the acidity of the stomach. Thus, a higher intake of more
collagen will not produce a greater quantity of collagen; your body will make as much as it needs. Synthesis Of Collagen Fibres:
-Collagen starts out as any ordinary polypeptide, as a linear sequence of amino acids, at which point it
will form with two other collagen polypeptides a triple helical structure. The triple helix then bundles
with other triple helices in order to form what are known as collagen fibrils. After that, bundles of fibrils
will aggregate together and create a collagen fibre, the native state collagen in the body.
Collagen Helical Folding (Procollagen Triple Helix):
- In the construction of the collagen triple helix, it is the presence of so much proline in the residue
sequence of collagen that prevents it from forming a typical alpha helix. Instead, it forms a polyproline
type II helix as the first step in folding. This kind of helical structure is more extended than the alpha
helix, and possesses no internal hydrogen bonding (due to amide oxygen and nitrogen atoms