4.7 - Amino Acids, Peptides and Proteins

α-amino acids

Has the -NH2 group bonded to the carbon atom that is next to the -COOH group

General formula of α-amino acids

Zwitterions

Dipolar form of an amino acid where a H+ is lost from the carboxylic acid group and gained by the lone pair of electrons on the nitrogen of the amino group

State of α-amino acids at room temperature

White crystalline solids

General formula for a zwitterion

Amphoteric nature of α-amino acids

Due to zwitterionic nature, they can react as both an acid and a base

Dissolved in acidic solution —> gain a proton

Dissolved in alkali solution —> lose a proton

Isoelectric point

pH at which zwitterions exist. Value varies depending on the amino acid

Amino acids in acidic solution

pH of solution is lower that the amino acid’s isoelectric point

Acts as a base

Accepts a proton

Formula for amino acid in acidic solution

Amino acids in alkaline solution

pH is greater than the isoelectric point Acts as an acid

Loses a proton

Equation for amino acids in acidic solution

Equation for amino acid in alkaline solution

Bonding between zwitterions

Ionic nature so form strong ionic bonds between positive ion one one and negative ion on adjacent —> higher than expected melting point

Melting temperature of amino acids

Ionic nature so form strong ionic bonds between positive ion one one and negative ion on adjacent —> higher than expected melting point

Solubility of amino acids

Generally soluble in water

Insoluble in non-polar organic solvents

Reflects presence of zwitterions

Extent of solubility in water depends on size and nature of R group

Condensation reaction

a reaction in which two molecules are joined to form one molecule. A smaller molecule is lost in the reaction, often water.

Formation of a dipeptide

2 amino acid molecules join together in a condensation reaction. Can be the same one or different

Bond formed in dipeptides

Between carboxyl group of one and amino group on the other. Peptide bond/amide linkage

Product formed from condensation of two amino acids

Dipeptide/amide

Number of dipeptides formed from condensation of two of the same amino acid

One

Number of dipeptides formed from condensation of two different amino acids

Two

Reaction to produce a dipeptide

Condensation between two amino acids

Polypeptide

A long chain of many condensed amino acids joined together by peptide bonds. Can lead to protein formation

Formation of polypeptides

Combination of more than 2 amino acid molecules

Formation of proteins

From polypeptide chains

Primary structure of protein

Sequence of amino acids making up the protein chain

Secondary structure of protein

How parts of the protein can fold up to form an α-helix that is held in place by hydrogen bonds or a β-pleated sheet in which the amino acids form a shape like a piece of paper stabilised by hydrogen bonds between amino acids in different polypeptide chains

Bonds in secondary structure of protein

Hydrogen bonds between amino acids in different polypeptide chains

Tertiary structure

Refers to the protein as a whole. Way in which the α-coils or β-pleated sheets of the protein fold with respect to each other

• Hydrogen bonds

• Disulfide bridges (-S-S-)

• Salt bridges/ionic interactions between RCOO- and RNH3+

Bonding in the tertiary structure

• Hydrogen bonds

• Disulfide bridges (-S-S-)

• Salt bridges/ionic interactions between RCOO- and RNH3+

Role of proteins in living systems

Essential component of a healthy diet

Necessary for;

• Structural functions (rigid structures)

  • Collagen in cartilage

  • Keratin in nails, hair and feathers

• Enzymes + biological catalysts

  • Amylase in human salvia which catalyses the hydrolysis of starch into sugars

• Hormones

  • Insulin

Examples of proteins in organisms

• Structural functions (rigid structures)

  • Collagen in cartilage

  • Keratin in nails, hair and feathers

• Enzymes + biological catalysts

  • Amylase in human salvia which catalyses the hydrolysis of starch into sugars

• Hormones

  • Insulin

Examples of proteins in structures

• Structural functions (rigid structures)

  • Collagen in cartilage

  • Keratin in nails, hair and feathers

Examples of proteins as enzymes

• Enzymes + biological catalysts

  • Amylase in human salvia which catalyses the hydrolysis of starch into sugars

Examples of proteins as hormones

Insulin

Examples of proteins in commercial use

• Rennin in cheese making

• Protease in detergents to remove protein stains