B1.2: Proteins

How many amino acids do humans have?

20

How many amino acids can be made, and how many need to be taken in?

11 made, 9 taken in

Common structure of amino acids

Central carbon linked to NH2 and COOH groups, one hydrogen atom, variable group (R)

Polypeptide examples

Haemoglobin, keratin, insulin

Peptide bonds

Condensation reaction

Amino acid structure

Central carbon liked to amine group, carboxyl, r-group, hydrogen atom

Essential amino acids

Cannot be synthesised by the organism, must be taken up in the diet

Obtaining essential amino acids

They are all produced by plants during photosynthesis, are passed through the food chain

Peptide/polypeptide def.

An unbranched chain of amino acids, short or long (respective)

Denaturation

Heat or excessive pH change causes intramolecular bonds in peptides to break, changing the shape of the protein, which defines its purpose.

Denaturation reversability

At mild disruption to optimum pH/temperature, only hydrogen bonds are broken causing denaturation. Returning to optimum pH/temperature would allow bonds to be re-established.

With excessive disrpution, covalent bonds are irreversibly broken, and returning to optimum pH/temperature would not allow these bonds to be re-established.

What defines the function of a protein?

Its shape

Outline structure of proteins

• Formed from amino acids

• Linked together by peptide bonds

• May consist of one or more polypeptides

• Have a specific shape/conformation/folding

• Shape determines function

Outline protein synthesis in the cell

• Proteins are produced when genes are expressed, they are coded for in DNA

• Genetic codon in DNA = 3-base sequence

• 1 DNA codon = 1 amino acid

• Codons are transcribed by mRNA

• mRNA exits the nucleus

• mRNA codons are translated by tRNA into polypeptides (chains of amino acids)

• Polypeptide/protein synthesis finalised at the ribosome

• Sequence of amino acids decided by order of DNA bases

• Proteins vary depending on type and order of amino acids

Outline enzyme denaturation

• Change in pH or temperature causes denaturation

• Intramolecular bonds are broken

• Causes change to protein shape

• Protein becomes denatured

• Function of protein is halted/impaired by damage to shape

Enzyme denaturation consequences

• Enzyme loses structure, active site changes shape

• Active site damaged/changed = substrate unable to bind to active site (as efficiently)

• Impaired/halted rate of reaction

Polypeptide vs protein

Polypeptides = chains of amino acids, proteins = complex structures made of one of more polypeptides.