proteins

What elements do proteins contain?

Carbon, hydrogen, oxygen, nitrogen + sometimes sulfur

Define amino acids

The sub units (monomers) from which proteins (polymers) are formed

What is the general structure of an animo acid?

• COOH - carboxylic acid group

• R side group (represents the rest of the molecule)

• NH₂ - amine group

How many commonly occurring amino acids is there?

20 different ones

What makes the 20 amino acids different?

All amino acids have the same amine group + carboxylic acid group but what makes them different is there carbon-containing side / R groups which give them their different properties

Give some examples of properties of amino acids that their R group give them

• Acidic (-ve charge)

• Basic (+ve charge)

• Polar (hydrophilic)

• Non-polar (hydrophobic)

* +ve and -ve side chains can be attracted to each other

* hydrophobic groups attract each other + repel water

How are amino acids joined together?

By peptide bonds (-CONH-) which are formed during a condensation reaction, eliminating a molecule of water

→ 2 amino acids form a dipeptide

What reaction splits a dipeptide apart?

Hydrolysis

What is a polypeptide?

Many amino acids joined together in a chain

A functional protein may consist of one or more ____

polypeptide

How can proteins be hydrolysed?

• By heating with acid

• Using enzymes → proteases

Proteins vary in the ___, ___ and ____ of amino acids they contain → this produces a vast number of different protein molecules + are classified according to their ____

number, type and sequence

structure

Define the primary structure of proteins

It refers to the sequence of amino acids in the polypeptide chain → this sequence determines the specific shape of the protein

What does the secondary structure represent?

The folding / coiling of the polypeptide chain as a result of hydrogen bonding between the amino acids → secondary structures include the alpha helix + beta pleated sheet

ALPHA HELIX

• all N-H bonds on same side of protein chain

• spiral shape

• H-bonds parallel to helical axis

BETA PLEATED SHEET

• N-H + C=O groups alternate from one side to another

What is the tertiary structure formed by?

Further folding + coiling of the secondary structure due to hydrogen bonds, ionic bonds + disulphide bonds → these bonds form in places determined by the sequence of amino acids in the primary structure, they form between R groups of amino acids

Why do proteins have such a wide variety of roles?

Because tertiary structures are specific + unique e.g tertiary structure of an enzyme determines the shape of it active site + its precise function

TERTIARY STRUCTURE

Where do hydrogen bonds form?

Ionic bonds form between what?

Why do hydrophobic regions form?

Disulphide bond form as a result of what?

• Hydrogen - between R groups of polar amino acids

• Ionic - between positive + negatively charged side chain of basic + acidic amino acids respectively

• Hydrophobic - because hydrophobic side chains exclude water + are attracted to each other

• Disulphide - an oxidation reaction between two cysteines

What are globular proteins?

Soluble proteins that consist of a highly folded + coiled polypeptide chain to produce a compact, complex tertiary structure → include enzymes + antibodies

What is quaternary structure?

When proteins consist of more than one polypeptide chain

The polypeptide chains are held together by ionic, hydrogen and sometimes disulphide bonds

• Can have non-protein groups involved in the quaternary structures

How can a small change in primary structure lead to a non-functional protein?

Change in sequence of amino acids means H-bonds form in different places, altering secondary structure → further H-bonds, ionic bonds, disulphide bridges form in different places → different tertiary structure

* if this protein is an enzyme : shape of active site is altered → no longer complementary to substrate → cant catalyse reactions

Define denaturation

An alteration in the tertiary structure of a protein → this loss of a 3-dimensional shape of the protein is often irreversible + protein is no longer functional

What can denaturation be caused by?

• Breaking of hydrogen + ionic bonds

• High temperatures above optimum → break H-bonds

• Extreme changes in pH → break ionic bonds

• Heavy metals → bind to a site on protein, changing its shape

• Detergents → disrupts hydrophobic interactions

• Reducing agents → breaks disulphide bridges

What bonds aren’t broken at the temperature that break hydrogen + ionic bonds?

Disulphide bonds

* In some proteins, disulphide bonds can remain unbroken at 70C

How to test a sample for protein

• Add Biuret reagent to sample

• A purple / lilac colour indicates protein is present

• If solution remains blue, no protein is present