4.7- Amino acids, Peptides and Proteins

What functional groups do amino acids contain?

Amine (NH2) and Carboxyl (COOH)

What is the naturally occurring formula for amino acids?

H2NCH(R)COOH

What does the R represent?

The variable group

What do amino acids have?

A chiral centre

Name H2NCH(C2H5)COOH

2-aminobutanoic acid

Name H2NCH(C3H7)COOH

2-aminopentanoic acid

How would you draw 2-amino-3-phenyl propanoic acid?

C6H5CH2CH(NH2)COOH

What can amino acids form and therefore act as?

Zwitter ions, can act as acids and bases (pH buffer)

At what pH do zwitter ions exist?

Isoelectric point

What may different isoelectric points be because of?

Different variable groups

What structure will HOCH2CH(NH2)COOH have in a low pH?

HOH2CCH(NH3+)COOH

At the isoelectric point?

HOH2CCH(NH3+)COO-

High pH?

HOH2CCH(NH2)COO-

Explain how they act as zwitter ions?

There is an internal transfer of a hydrogen ion from the -COOH group to the -NH2 group to leave an ion with both a positive and negative charge

Does a zwitter ion have an overall electrical charge?

No

How do amino acids have a high melting points and are solids?

Because of the ionic electrostatic forces of attraction between the zwitter ions

How are they more soluble?

Ion-dipole interactions between the zwitter ions and the water molecules

What happens when amino acids react with strong acids such as HCl?

They will form a salt

Eg?

-Cl+H3NCH(R)COOH

What will happen when an amino acid reacts with NaOH?

A salt and water will form

H2NCH(R)COOH + NaOH ->

H2NCH(R)COO-Na+ + H2O

Amino acid + alcohol?

Will form an ester and water

What in the presence of?

Concentrated sulfuric acid

Equation?

H2NCH2COOH + C2H5OH + H+ -> +H3NCH2COOC2H5 +H2O

What are amides the product of?

The reactions of acid chlorides with ammonia and amines

What can two amino acids react with each other to form?

A dipeptide

What is eliminated?

A water molecule

What is this type of reaction called?

A condensation reaction

What does the bond look like?

-C=O -N-H -

What can amino acids form?

Polypeptide chain

How to convert drawing of amino acid to polymer?

H2NCH(R)COOH -> [-NH -CH(R) -C=O-]

How can the peptide link be broken?

By heating under reflux in strong acid or strong alkali

H2NCH(R)CONHCH(R)COOH + H2O ->

H2NCH(R)COOH + H2NCH(R)COOH

What would happen in acidic conditions?

+H3NCH(R)COOH + +H3NCH(R)COOH

amino acids are produced in their cationic form

What happens in alkaline hydrolysis?

They are broken down to form carboxylates (eg their sodium salts) by heating with aqueous alkali

Define primary structure

The sequence of amino acids in a polypeptide chain

What bonds are in the primary structure?

Peptide

Define secondary structure

The twisting and folding of a polypeptide chain into an alpha helix or a beta pleated sheet. Held by peptide and hydrogen bonds

What are the hydrogen bonds between?

The N-H hydrogen atom of an amide group and the C=O oxygen atom of another amino acid

What is the beta pleated sheet stabilised by?

Hydrogen bonds between amino acids in different polypeptide chains

Define tertiary structure

further folding and coiling of the secondary structure to give a specific 3D shape. Held in place by ionic, disulphide, hydrogen bonds and hydrophobic interactions.

How are proteins used in living systems?

Biological catalysts eg. amylase catalyses the hydrolysis of starch into sugars

Structural function in living organisms eg. keratin in hair

Hormones eg insulin in blood glucose regulation