Proteins – p. 25http://www.wisc-online.com/objects/index_tj.asp?objID=AP13304

Cells contain thousands of different proteins, each performing a specific task.

Examples: Enzymes, immunoglobulins (used in immune response), haemoglobin (oxygen carrier in blood), keratin (In hair and fingernails), fibrin (helps blood clot), collagen (in bones, skin, tendons).

Consist of long chains of amino acids (amino acid polymers) folded into specific 3-D shapes.

Amino AcidsThere are 20 different amino acids each with

a different side chain.The common structure of all amino acids is:

- A central Carbon atom to which are attached an amino group, carboxyl group, a hydrogen atom and side chain (R).

Amino Acid general structure

PropertiesContain acidic (carboxyl) and basic (amino)

functional groups.

Can be polar, non-polar, or charged (acidic or basic) depending on the nature of their side chains.

How proteins are formedProteins consist of chains of amino acids

(polypeptides) that are twisted and coiled into a specific shape or conformation.

Polypeptides are made in the cytoplasm of cells during protein synthesis.

The amino acids in the cytoplasm come from the food eaten or are made from simpler compounds.

How proteins are formed con’tOf the 20 amino acids, 8 cannot be made

from simpler compounds and must be obtained from diet. These are essential amino acids.

During protein synthesis, bonds called peptide bonds are formed between amino acids by a condensation reaction.

http://www.johnkyrk.com/aminoacid.swf

Protein StructureThe structure of a protein determines its

function! Eg. A structural protein will be roughly linear in shape and will from strands or sheets (fibrous proteins)

Many enzymes and other functional proteins are globular (rounded, spherical shape).

There are four levels of structure:

1. PrimaryThe unique sequence of amino acids in a

polypeptide chain.

Determined by nucleotide sequence in DNA.

If one amino acid is out of order, the entire protein could be useless!

2. SecondaryAs the protein grows in length, it coils and

folds creating the protein’s secondary structure.

In some polypeptides, repeated H bonds cause it to twist into a coil called an α-helix. (Eg. -keratin in hair, sections of the enzyme lysozyme in saliva, sweat and tears)

When two parts of the chain lie parallel to each other and are bonded by H-bonds, β-pleated sheets are created. (Eg. Proteins in spider webs)

3. TertiaryThis structure results from the nature of the

different amino acids interacting with the environment.

For example, polar amino acids are attracted to water so they interact with it while non-polar amino acids will congregate in the centre, away from the water.

This causes the polypeptide to fold into a certain shape – the tertiary structure.

http://www.johnkyrk.com/aminoacid.swf

4. QuaternaryShape of a protein resulting from 2 or more

polypeptides coming together. Eg. Collagen, keratin, haemoglobin

-Sometimes a chaperone protein may help the protein fold into its quaternary structure.

Collagen

Haemoglobin

Denaturing ProteinsThe 3-D shape of a protein can be destroyed

or changed due to changes in temperature, pH, ionic concentration, etc. This is called denaturation.

When the structure is destroyed or changed, the proper function cannot be carried out.

Sometimes protein denaturation can be useful. We do it every day when we cook meat or curl/straighten hair.

QuestionsPage 28, #19-24Bring a device with internet access for a quiz

next class!!