Proteins

What are proteins made from?

Monomers - amino acids. Dipeptide is formed when two amino acids join together, polypeptide is more than two. Proteins are made up of one or more polypeptides.

Amino acid structure

A carboxyl group (COOH) , amino group (NH2) and R group (variable group) attached to a carbon atom.

R groups usually contain C (glycine exception - R group one H atom)

All living things share bank of 20 amino acids. The R group is what differentiates them.

Polypeptide formation

• Condensation reactions - water molecule released

• Peptide bonds between amino acids

• Reverse reaction hydrolysis

Primary Structure

The sequence of amino acids in the polypeptide chain

Secondary Structure

• Hydrogen bonds form between amino acids in the chain

• This makes it: coil into an alpha helix OR fold into a beta pleated sheet

Tertiary structure

• coiled or folded chain is coiled or folded further

• more bonds form between different parts of polypeptide chain (hydrogen and ionic - attraction between positive and negative charges of different parts of the molecule)

• Disulfide bridges also form - when two molecules of amino acid cysteine come close together - a sulphur atom from each acid bond.

• IF protein is single polypeptide chain, tertiary is final 3D structure.

Quaternary structure

• Several polypeptide chains held together by bonds (ionic, covalent)

• The final 3D structure (haemoglobin, insulin, collagen)

• Protein’s final 3D structure

What are the main protein functions?

• Enzymes

• Antibodies

• Transport proteins

• Structural proteins

Enzymes

• roughly spherical - tight folding of polypeptide chains

• soluble - often roles in metabolism - i.e breaking down or synthesising large food molecules

Antibodies

• immune response involvement found in blood

• two short (light) polypeptide chains two long (heavy) polypeptide chains bonded together

• have variable regions where amino acids sequences vary greatly

Transport proteins

• i.e channel proteins in cell membranes (contain hydrophobic and hydrophilic amino acids, which cause protein to fold up and form a channel

• transport molecules and ions across membranee s

Structural proteins

• physically strong

• long poly peptide chains lying parallel to each other with cross links between them

• i.e keratin, collagen

Fibrous proteins

The Quaternary structure is made up of three polypeptide chains -(whose tertiary structure is tightly

wound helix) wound together around each other in a helix fashion like in a rope.

o This gives them specific properties – namely strength

o A good example is collagen or tendons because they need to be strong to hold muscle to bone.

Globular proteins

these carry out metabolic functions – like enzymes which catalyse reactions in the body.