Proteins

Function of Proteins

• There are thousands of different proteins that are produced by the cells of your body.

• Next to water, your body is composed mostly of protein.

Function of Proteins

• 1. Structural– Building and repair of tissue• Ex. Keratin (hair and nails)• Ex. Collagen (support to ligaments, tendons and skin)

Function of Proteins

• 2. Hormones– Messengers that are produced in one place in the

body, but cause a chemical reaction to occur in another place.• Ex. FSH (follicle stimulating hormone) is produced in

the brain, but causes ripening of the egg in the ovaries.

Function of Proteins

• 3. Transport of molecules– In the blood• Ex. Haemoglobin in red blood cells to carry O2

– Across the cell membrane

Function of Proteins

• 4. Antibodies– Found in blood and body fluids and work to

destroy foreign sunstances• Ex. White blood cells

Function of Proteins

• 5. ENZYMES!!!– Proteins that speed up chemical reactions• Ex. Amylase• Ex. Pepsin

Function of Proteins

• There are also proteins for the movement of cells like actin that works in cells and myosin that works in muscles.

• Lipoproteins also exist and they carry lipids like cholesterol in the blood.

Protein Structure

• Proteins are polymers of amino acid monomers.– They’re basically chains of amino acids

• All amino acids contain the elements C, H, O, N and sometimes S.

• All proteins are synthesized from a pool of 20 amino acids that are found in the cytoplasm of cells.

Amino AcidsEssential (dietary proteins) Non-essential

- Not made by the body - Made by the body

- Must be present in our foods - Not necessary from our diet

- All 8 are found in meat, milk, eggs (these foods are called complete proteins)

- The body can transform one amino acid into another

Amino Acid Structure

• Every amino acid is made up of the same 3 basic parts.– First group: Amino group

Amino Acid Structure

• Last group: Acid group (carboxyl)

Amino Acid Structure

• Middle group: “R” Group (Remainder of the molecule)

• So when you put them all together you have an amino acid monomer.

Amino Acid Structure

Amino Acid Structure

• It is the “R” group that differentiates all 20 amino acids from each other.

• Each of the 20 amino acids has a different “R” group.

• Some specific examples…

Amino Acid Structure

• Glycine

“R” Group

Amino Acid Structure

• Lysine

Amino Acid Structure

• Methionine

Peptide Bonds

• A peptide bond is a bond that joins one amino acid to another.

• When only 2 amino acids join together we call the resulting molecule a dipeptide.

Practice

• Show the equation for the dehydration synthesis and hydrolysis of the following peptides:– 1. glycine + lysine– 2. threonine + methionine

Levels of Protein Organization

Dipeptides

• A dipeptide is formed when any 2 amino acids are joined together by dehydration synthesis and broken down by hydrolysis.

• The bond between these amino acids is called a peptide bond.

Proteins

• If there anywhere from 50-2000 amino acids in the bond, it is now called a protein or a polypeptide.

• Each polypeptide has a different sequence of amino acids, and thus a different sequence of “R” groups.

Levels of Protein Structure

• Primary structure– A linear sequence of amino acids joined by

covalent peptide bonds between different amino acids.• Ex. Hair

Levels of Protein Structure

• Secondary structure (two types)– Alpha helix (right hand spiral) has the chain coiled

like a telephone cord.• Ex. keratin

– Pleated sheet has the chain folded.• Ex. The proteins in silk

– These are both due to weak hydrogen bonds between different amino acids in the protein.

Levels of Protein Structure

• Tertiary structure– The helix folds back unto itself into a 3-

dimensional globular shape.– These are called globular proteins.– They occur when there are covalent, ionic or

hydrogen bonds between different “R” groups.• Ex. enzymes

Levels of Protein Structure

• Quaternary structure– Occurs when 2 or more globular proteins join and

fold around each other to form 1 complex protein.• Ex. Haemoglobin – 4 polypeptides joined together.

Changing the Shape of a Protein

• Any irreversible change in shape in a protein due to some external stress is called denaturing.– Ex. Adding lemon juice to milk to make it curdle.– Ex. Cooking eggs to make the white become

opaque.

Changing the Shape of a Protein

• Causes of denaturing– 1. Protein has been heated to an extreme

temperature like cooking.– 2. pH has been changed like lemon juice.– 3. Protein has been exposed to heavy metals such

as Hg, Pb or Cd.

Changing the Shape of a Protein

• Results of denaturing– The denatured protein undergoes a change in

shape and can no-longer perform its function.• Ex. Enzymes when denatured cannot work because the

substrate can no longer bind to the active site.• Ex. Denatured antibodies cannot lock onto its

corresponding antigen