PHAR 202 - Quaternary Structure

What is a quaternary structure

• proteins that are composed for more than 1 polypeptide chain

What is the molecular mass of proteins in quartenary structures

• the spatial arrangements of these subunit arrangements

larger than 100kD

Why is it useful for proteins to be made of multiple subunits instead of one large polypeptide?

Smaller subunits are easier to fold and manage.

How do multi-subunit proteins deal with defective components?

A faulty subunit can be replaced without remaking the whole protein.

What advantage does assembling proteins at different sites give?

Subunits can be synthesized in one place and assembled in another, giving flexibility.

How is it more efficient to increase protein size through subunit association?

By reusing identical subunits instead of coding for one huge polypeptide.

Where are the active sites often located in multimeric enzymes?

At the interfaces between subunits.

How do multiple subunits help regulate protein activity?

They provide a structural basis for regulation

What is a multimeric protein

a protein made of multiple individual protein subunits (polypeptide chain)

A multimeric protein may consist of identical or non-identical polypeptide chains, called

homooligomers

heterooligomers

How subunits stick together

when they associate, they bury 1000 to 2000 Ų, at least ~600 Ų surface area that would other wise be exposed to solvent

• this results in them looking like the interior of a single subunit

What are the similarities between two subunits sticking together and a single subunit?

• closely packed non-polar side chains

• hydrogen bonds

• interchain disulfide bonds

What are the differences between two subunits sticking together and a single subunit?

• protein-protein interfaces

  • less hydrophobic bcs fewer hydrophobic patches

• Hydrogen bonding patter

  • 77% happen between side chains (INTERSUBUNIT)

  • 68% happen between backbond (INTRASUBUNIT)

Around …..% protein-protein interfaces contain salt bridge

56%

Salt bridges contribute to the specificity and stability of

subunit associations

Hydrophobic interactions and spaces offer ……… environments

Low dielectric constant