Protein Structure & Function

What are proteins?

What are proteins?Proteins are made up of C, H, O, N and some

S and PProteins are the building blocks of lifeThere are millions of different proteinsProteins are the most abundant molecules in

cellsProteins make up more than 50% of a cell’s

dry mass

What do proteins do?Proteins perform a wide range of biological

functions:• As enzymes they catalyse reactions.• Carrier proteins transport molecules across

membranes.• Antibodies defend against disease.• Structural proteins support cells and tissues.• Hormones transmit information.• Transport proteins such as haemoglobin carry

oxygen.• Contractile proteins enable muscles to

contract.

How do proteins do all this?Proteins can carry out all these different

functions because each different protein has a specific molecular shape which enables the protein to do its job.

It is structure of a protein that allows it to carry out its function.

So, what goes into a protein?

So, what goes into a protein?Proteins are polymer molecules.The monomer molecules making up proteins

are called amino acids.There are 20 different naturally occurring

amino acids.There are over 100,000 combinations of

amino acids forming known proteins.

What is an amino acid?All amino acids have the same general

structure:• A carboxyl group (-COOH)• An amino group (-NH2) attached to a C atom• A variable group called R

It is the R group that differs from one amino acid to another

Are all amino acids ‘equal’?No, of the 20 naturally occurring amino acids 8

are known as ‘essential amino acids’. These 8 cannot be synthesised by the body and must be obtained from the diet.

The remaining 12 can be synthesised by the body.

Joining amino acids togetherWhen amino acids join together, they do so by a

condensation reaction.This means one water molecule is removed,

using the OH group from the carboxyl group of one amino acid, and one H from the amino group of another.

The resulting bond is called a peptide bond.

PeptidesTwo amino acids joined together form a

dipeptide.Three amino acids joined together form a

tripeptide.A polypeptide is made up of many amino

acids joined together.When a polypeptide bonds with other

polypeptides it forms a protein containing thousands of amino acids.

Proteins have 4 structural levels.Proteins are big, complicated, 3-dimensional

molecules.The structure is described in four ‘levels’:• Primary• Secondary• Tertiary• Quaternary

Primary structureThe primary structure of a protein is the

sequence of amino acids in the chain.The primary structure determines the

eventual shape of the protein, hence its function.

Secondary structureThe amino acids in the primary structure of a

protein do not lie flat and straight.Hydrogen bonds form between the amino

acids in the chain.This makes the protein coil into an a helix or

fold into a b pleated sheet.This is the secondary structure.

Tertiary structureThe coiled or folded chains often coil or fold

further.More bonds form due to interactions between

theR-groups of the polypeptide chain.

This is called the tertiary structure.For proteins formed from a single

polypeptide chain this is the final 3D structure of the protein.

Quaternary structureSome proteins are made up of several

polypeptide chains held together by bonds.The quaternary structure is how these chains

are put together.The best known example is haemoglobin,

which is made of four polypeptide chains bonded together. For proteins such as haemoglobin, the quaternary structure determines the final 3D structure.

Haemoglobin

Protein bondsThe four structural levels in proteins are held

together by different bonds:• Peptide bonds (primary)• Hydrogen bonds (secondary and tertiary)• Ionic bonds (tertiary)• Disulphide bonds (tertiary)• Hydrophobic and hydrophilic interactions

(tertiary)Quaternary structure depends on the tertiary

structure of the individual polypeptides, and so is influenced by all these bond types.

Types of proteinThere are two different types of protein and

they are different shapes. The shape of a protein molecule is related to its function.

• Globular proteins – these are round, compact and easily soluble so they can be transported in fluids. Examples are haemoglobin and enzymes.

• Fibrous proteins – these are tough and rope-shaped. They tend to be found in connective tissues such as tendons. Collagen is an example of a fibrous protein.

HaemoglobinHaemoglobin is a globular protein.It’s structure is curled up so that hydrophilic

side chains face outwards and hydrophobic side chains face inwards.

This makes haemoglobin soluble and therefore good for transport in the blood.

CollagenCollagen it made of three polypeptide chains,

tightly coiled in a strong triple helix.The chains are interlinked by strong covalent

bonds.Minerals can bind to the triple helix to increase

its strength.

PlenaryName the two groups found in all amino acid

molecules.Name the bond that joins amino acids

together in proteins.Name the four types of bond that determine

the structure of a protein.Name the four structural levels of a protein.