Synthesis of proteins

Objectives

To learn about the structure of proteins

To understand how the information from the DNA is used to synthesise proteins;

We will not deal with the replication of DNA here

What are proteins?

Chemicals that perform many important roles in the body

Made up of monomers called amino acids

Amino acids are carboxylic acids with amine groups attached

Dissolved in water they are ionised and the H+ ion dissociates into the water.

20 different amino acids present in proteins in humans (there are many others)

Types of amino acids

Chemical features of amino acids dependent on the side chain (R)

R can be alky, hydroxyl, thiol (-SH), amino, sulphur or aromatic groups or rings containing C and N

Non-polar amino acids have hydrocarbon side chains and are hydrophobic.

Polar amino acids have ionic side chains and are hydrophilic

Which do you think will dissolve more easily?

The amino acids

Class of protein

Function in body Examples

Structural What we are made of Collagen in tendons and cartilage. Keratin in hair, skin wool and nails

Contractile Movement of muscles Muscle fibres Myosin and actin

Transport Carry essential substances through body

Haemoglobin carries oxygen. Lipoproteins transport lipids

Storage Store nutrients Casein stores protein in milk, Ferritin stores iron in liver

Hormone Regulates body Insulin etc.

Enzyme Catalyze biochemical reactions in the cells.

e.g. sucrase catalyses the hydrolysis of sucrose into glucose and fructose

Protection Recognise and destroy foreign susbstances

Immunoglobulins stimulate immune response.

Structure of proteins

Primary structure:

Bonds between amino acids are called peptide bonds

Combinations of amino acids are called polypeptides.

A polypeptide chain of more than 50 is called a protein

This is the order in which the amino acids are combined on the RNA strand.

The first protein to have its primary structure determined was insulin which comprises 2 polypeptide chains.

Chain A has 21 amino acids, Chain B has 30.

Polypeptide chains held together by disulphide chains formed by the side chains of the cysteine amino acids.

Secondary structure

The forming of bonds between amino acids in different parts of the chain or on different chains leads to the folding or bending of the chains into a secondary structure.

Three commonest forms of secondary structure are called:

α helix: β- pleated sheet and the triple helix

Bonds

Alpha helix are held together by Hydrogen bonds between N-H and C=O groups at each turn of the helix.

Beta-pleated sheets held together by H bonds between polypeptide shapes.

Triple helix formed from 3 polypeptides woven together. When several triple helices wrap together they form fibrils Collagen is a triple helix. As we age more cross bonds form between the fibrils making it less elastic.

Tertiary structure

This is formed by repulsions and attractions between side chain groups of amino acids

These cause the peptide chains to twist and bend

Gives each protein a specific three dimensional shape

Stabilised by interactions between amino acid side chains (R) groups.

Tertiary structure determines the biological action of the protein

Changing tertiary structure with heat or pH alters causes it to denature.

Quaternary structure

A combination of two or more protein subunits to form a larger biologically active protein

E.g. Haemoglobin

Contains 4 polypeptide subunits

Each contains a Haem group containing and Fe2+ ion which binds to an oxygen molecule

Proteins plenary

1. What are proteins made of?

2. What is the purpose of contractile proteins and name them?3. What controls the secondary structure of a protein?

4. What controls the activity of a protein?

5. What is the purpose of haemoglobin?

6. Which non organic component does it contain?

The structure of DNA

This is the key!

DNA

De-oxyribosenucleic acid (RNA) Ribosenucleic acid

Made of subunits which scientists called nucleotides.

Each nucleotide is made up of a sugar, a phosphate and a base.

There are 4 different bases in a DNA molecule: adenine (a purine) cytosine (a pyrimidine) guanine (a purine) thymine (a pyrimidine)

The number of purine bases equals the number of pyrimidine bases

DNA contd.

The number of adenine bases equals the number of thymine bases

The number of guanine bases equals the number of cytosine bases

The basic structure of the DNA molecule is helical, with the bases being stacked on top of each other.

Protein Synthesis

http://www.youtube.com/watch?v=NJxobgkPEAo

The base pairs: Adenine to Thymine (uracil in RNA)Guanine to cytosine

Transcription and translation Transcription: The code in the DNA is

converted to a messenger RNA (mRNA) strand.

Takes place in the nucleus.

Translation: the process by which the message on the mRNA is converted to a protein: Takes place in the cytoplasm and uses ribosomes and transfer RNA (tRNA).

Transcription

These are joined into a strand by RNA polymerase and the transcribed strip hangs free from the DNA.

RNA polymerase causes complimentary RNA bases to match up with the DNA along the template strand;

This starts when RNA polymerase joins to a stretch of DNA called the promoter which defines the start of a gene.

To start the DNA helix has to be opened by breaking the hydrogen bonds between the nitrogenous bases;

It is the process by which the DNA code is translated to form mRNA;

It relies on the base pairs;

Transcription contd.

After some further processes the mature mRNA is transported through nuclear pores to the

cytoplasm for translation.

This stops RNA polymerase transcribing the DNA and the strip of mRNA now called an mRNA

transcript falls off.

Elongation of the mRNA strand continues until a stretch of DNA called the stop sequence is

reached.

Transcription diagram

Translation

Takes place in the cytoplasm;

Is how the mRNA code is translated into a protein;

It takes place at a ribosome

A sequence of nucleotides is translated into a sequence of amino acids which are linked to form a protein.

Translation continued

tRNA

Role of tRNA

tRNA molecules transfer amino acids to the ribosomes;

At least 1 tRNA molecule for each of the 20 amino acids;

One end is attached to an amino acid;

The other end to an anticodon which complements a 3 base codon on the mRNA

Write the anticodons which pair with these codons

Codons

GUC

CUU

CCU

GAC

Genetic code

There are 61 codons (sets of 3 bases) that code for amino acids

There are three different stop sequences;

There are 40 different tRNA molecules;

Some tRNAs can pair with more than one codon;

This protects against changes in the DNA base sequence and ensures the correct proteins are synthesised.

Each amino acid is coded for by bases

How are amino acids and tRNA joined

Ribosomes comprise two subunits: a small one and a large one.

This then travels to the ribosome.

These have an attachment site which is the correct shape for their particular amino acid (lock and key);

Amino acids are joined to the correct tRNA molecule by special enzymes

Protein synthesis at the ribosome

Peptide bearing tRNA moves to the next binding site.

Emptied tRNA is released.

Amino acid is added to peptide chain by a ribosomal RNA (also called a ribozyme);

A peptide bearing tRNA passes its peptide chain to the amino acid bearing tRNA

a chain of amino acids called a peptide is formed at the ribosome as it moves along the mRNA.

2 the large subunit has three binding sites for tRNAs

1. mRNA binds to the small subunit of the ribosome;

Translation diagram

Forming the chain

http://www.youtube.com/watch?v=5bLEDd-PSTQ

The three steps of translation

Initiation - brings the parts together for translation:

initiation factors, small and large ribosomal subunits, mRNA and initiator tRNA;

Elongation – Protein synthesis step when

polypeptide chain increases by one amino acid at a time. Elongation factors help bind

tRNA codons to mRNA codons at the ribosome. Translocation

happens where the mRNA moves forward and a new codon is ready to receive a new tRNA with a new amino acid to attach to the peptide

chain.

Termination – polypeptide separates from ribosome

mRNA complex. This happens at a stop codon. Freed

polypeptide can then develop its 3D structure.

Plenary

Write down a question based on this lesson.

Be prepared to answer it and to find an answer for someone else’s question.

Question and answer sessions in 4s

Then we’ll go round the class.