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Wednesday, January 09, 2008

Quaternary structure - folded structures associate to form homo – or hetero-dimers

046. Proteins are linear polymers of amino acids. They fold into compact structures. Sometimes, these folded structures associate to form homo – or hetero-dimers. Which one of the following refers to this associated form?

1. Denatured state

2. Molecular aggregation

3. Precipitation

4. Quaternary structure

Answer

4. Quaternary structure

Reference:

Harper 26th Edition Page 34

Lippincott 3rd Edition Page 20

Chaterjee 6th Edition Page 86

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Discussion

When a protein consists of two or more peptide chains held together by non covalent cross links it is referred to as the quaternary structure The quaternary structure complex is often called as the oligomer and each constituent peptide as monomer or subunit. The noncovalent association of a molecule of beta-2 microglobulin with the heavy chain of each class I histocompatibility molecule is an example

Explanation

1. Denaturation is Destruction of the natural three-dimensional structure of large biological molecules (proteins or DNA), eg through heat. In the case of DNA in particular, denaturation means the dissolution of the double strand into single strands

2. Molecular aggregation

3. Precipitation is the formation of a solid in a solution during a chemical reaction. This can occur when an insoluble substance is formed in the solution due to a reaction or when the solution has been supersaturated by a compound. In most situations, the solid forms ("falls") out of the solute phase, and sinks to the bottom of the solution (though it will float if it is less dense than the solvent, or form a suspension).

4. Quaternary structure are Complexes of 2 or more polypeptide chains held together by noncovalent forces but in precise ratios and with a precise 3-D configuration.

Comments

Ä The primary structure of a protein is its linear sequence of amino acids and the location of any disulfide (-S-S-) bridges.

Ä Secondary Structure : Most proteins contain one or more stretches of amino acids that take on a characteristic structure in 3-D space. The most common of these are the alpha helix and the beta conformation.

o Alpha Helix

§ The R groups of the amino acids all extend to the outside.

§ The helix makes a complete turn every 3.6 amino acids.

§ The helix is right-handed; it twists in a clockwise direction.

§ The carbonyl group (-C=O) of each peptide bond extends parallel to the axis of the helix and points directly at the -N-H group of the peptide bond 4 amino acids below it in the helix. A hydrogen bond forms between them [-N-H·····O=C-] .

o Beta Conformation

§ consists of pairs of chains lying side-by-side and

§ stabilized by hydrogen bonds between the carbonyl oxygen atom on one chain and the -NH group on the adjacent chain.

§ The chains are often "anti-parallel"; the N-terminal to C-terminal direction of one being the reverse of the other.

Ä Tertiary structure refers to the three-dimensional structure of the entire polypeptide chain.

Tips

The number of sub units may vary

Ä 2 subunits (dimmer) – CPK

Ä 4 subunits (tetramer) – Hemoglobin and lactate dehydrogenase

Ä 24 subunits – Apoferritin

Ä 72 subunits – Aspartate transcarboxylase

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