Chapter 5: Protein Structures

Primary Structure

Sequence of amino acids

Secondary structure

Repeated folded structures held together by hydrogen bonds; looks like an alpha helix and a beta pleated sheet

Tertiary structure

Overall shape of the polypeptide; determined by r group interactions, hydrophobic interactions, Van der Waals interactions, hydrogen bonds, and disulfide bridges between cysteines; at this stage the protein may be fully functional

Quaternary structure

Two or more polypeptide chains folded together; some examples include hemoglobin and collagen

Sickle cell disease

Inherited blood disorder resulting from one change in the amino acid sequence (primary sequence) of hemoglobin; normal cells have glutamic acid in the sixth position, but sickle cell patients have valine in that 6th position, causing molecules to crystallize and not be able to carry as much oxygen and clog blood vessels

What factors affect protein shape and folding?

Temperature, pH, and salt concentration

Denaturation

When proteins unravel and become inactive; this can be due to extremely hot temperatures or chemical solvents that can disrupt bonds or cause proteins to fold and expose hydrophobic regions

Is it possible to predict the final shape of proteins?

Not really. Scientists have yet to find a reliable method based on aa (amino acids determines) sequence alone

Chaperonins

“Chaperones” that assist proteins with folding by providing a safe, isolated environment

Prions (proteinaceous infectious particles)

Normal proteins folded incorrectly; do not appear to be species specific like viruses are

What are the main characteristics of prions?

Prions are extremely resistant to heat (making denaturation harder), radiation, digestive enzymes, and protein denaturing agents; this is why there is no sterilization method that is considered effective against prions

What do prions cause proteins to do?

They cause normal copies of the protein to fold improperly, which creates large chains of prion proteins and leaves tissues susceptible to destruction

Transmissible spongiform encephalopathy

Gives brain tissue a spongy look, and causes slow, progressive dementia; transmission requires contact with infected blood or tissues and has been found to be the cause of Creutzfeldt-Jakob Disease, Shy-Drager Syndrome, Bovine Spongiform Encephalopathy, and scrapie in sheep

X-Ray Crystallography

Used to determine a protein’s shape