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Transcript of Protein Structure
Name the four levels of protein structure and describe the structural attributes of each level.
Identify the two main types of protein secondary structure and discuss the key chemical bonds holding them together.
Estimate the relative location of an amino acid within a protein based on the amino acid’s chemical properties.
Explain the roles of the key forces stabilizing proteins.
Primary Structure: The amino acid sequence
Secondary Structure: Local structure of the polypeptide chain
Secondary structure is formed by hydrogen bonding between the carbonyl oxygen and amide hydrogen atoms in the polypeptide chain.
Tightly coiled polypeptide
Side chains of the amino acid residues extend outward
Formed by hydrogen bonding
Sheet-like structures comprised of beta strands
Strands can be distant in the protein structure
Hydrogen bonds form between adjacent strands
Two different orientations: parallel & antiparallel
Tertiary Structure: 3-D, folded and biologically active conformation of a protein.
Overall 3-D structure is formed from simpler elements (secondary structure, etc.)
Close packed; 75% of the interior is filled with atoms
A few general patterns describe the overall organization of proteins.
(human growth hormone)
(retinol binding protein)
Tertiary Structure - Examples
Forces Stabilizing Protein Tertiary Structure
The amino acid residues govern the spontaneous formation of a protein's structure.
Protein folding is strongly influenced by water solubility of the amino acid R-groups.
Tertiary Structure in Proteins
Triose phosphate isomerase is a dimer of identical subunits.
Hemoglobin is composed of four subunits of two types, alpha and beta. It is represented as alpha2 beta2.
Examples of Quaternary Structure
Quaternary Structure: a complex of two or more separate polypeptide chains that are held together by noncovalent interactions.
Hydrophobic R-groups orient inwardly.
Polar or ionized R-groups orient outwardly.
Some amino acids can be accommodated by aqueous or non-aqueous environments.
Secondary & tertiary structure
Tendency to exclude water
The most intricate degree of organization considered to be a single molecule - higher levels of organization are multimolecular complexes.
Individual subunits alone are generally biologically inactive.
Note: If covalent links exist (such as disulfide bridges), then the structure is generally not considered quaternary, but this distinction is still debated.
Myoglobin – one subunit (chain), thus only tertiary structure;
Hemoglobin – two alpha and two beta subunits, thus quaternary structure.
Not all proteins have quaternary structure. Requirements include:
2 or more peptide chains;
Often observed to be subunits in a multi-subunit protein;
Can be different or identical subunits
Usually symmetrical arrangements
Subunits are held together by the same types of interactions that stabilize the tertiary structure of proteins.
2 subunits – dimer
3 subunits – trimer
4 subunits – tetramer
With an appropriate understanding of the material in this presentation, one should be able to...