Processes SL2.4, 2.6, 2.7

2.4: Proteins

Polypeptides

• Chains of amino acids joined together by peptide bonds

• Result of condensation reactions (also know as translation).

• How many amino acids are in a dipeptide?

Human insulin

Contains two polypeptide

chains

-

Titin

One of the largest polypeptides found so far

Found in muscles

Contains 34, 350 amino acids in humans.

In mice there are 35, 213 amino acids!

R-groups

You do not need to memorise the different R-groups.

BUT, you do need to know that amino acids vary due to the R-group.

There are 20 different amino acids, and they can be categorised based on the R-group. Pg. 89, table 1.

Patterns, trends & discrepancies…

Most organisms on Earth use the same 20 amino acids to form proteins.

Theories as to why include:1. These 20 amino acids were the ones

produced when life first developed. 2. Natural selection: these are the most

favourable amino acids for making proteins.

3. Common ancestry

Diversity• A dipeptide contains two amino acids. These can

be any combination of the 20. • A ribosome can link any of the amino acids.

Therefore the possibilities can be calculated by:• 20n (n = number of amino acids in a polypeptide

Number of amino acids

Number of possible amino acids sequences

1 201 20

2 202 400

3 203 8,000

4 204 160,000

The ‘open reading frame’

• This is the section of a DNA strand containing genes which code for a polypeptide.

• Each amino acid is coded for by 3 bases

• Therefore, in theory, a polypeptide with 400 amino acids would require a gene with a base sequence of 1,200 bases.

Lysozyme

• 1 polypeptide• Nasal mucus, tears. Kills bacteria by

digesting the substance in the cell wall.

Integrin

• Two polypeptides• Membrane protein

Collagen

• Three polypeptides• Structural protein (tendons, ligaments,

skin & blood vessels)• High strength, limited stretch.

Haemoglobin

• Four polypeptides• Oxygen binding

protein in RBC.

The Conformation of proteins

• Polypeptides do not remain in long chains. Attractions/bonding between the R-groups can lead to four basic structures.

FunctionsCatalysis (enzymes to catalyse reactions)

Immunity (antibodies)

Muscle contraction DNA packingCytosketeton (structure within membrane)

Recepetors (binding sites on membrane)

Tensile strengthening (fibrous proteins in skin)

Hormones (insulin, FSH etc.)

Blood clotting Membrane transportTransport (including nutrients & gas)

Cell adhesion

Examples to know

1. Rubisco2. Insulin3. Immunoglobulin4. Rhodopsin5. Collagen6. Spider silk

Proteomes• The total of all the proteins produced by a

cell, a tissue or an organism.

Gel electrophoresis is used to identify the proteins in a sample – florescent markers are attached to antibodies for specific proteins.

Proteomes vary, because different cells produce different proteins. The proteome for each individual is unique.

Assessment

• Design an experiment to investigate the effect of either temperature or pH on egg albumin.

• You will conduct this experiment next lesson. You will be assessed on your data collection & processing skills ONLY.