Amino Acids and Proteins Flashcards
What are the biological roles of proteins?
- Transport e.g. o2 on haemoglobin
- Storage e.g. Fe2+ in complex with ferritin in liver
- Enzyme catalysis: allows reactions to take place at 37ºC instead of as high as 200ºC
- Motion: e.g. muscle, intestinal flagella
- Skeletal support e.g. collagen in bone and skin
- Immune protection e.g. antibodies
- Nerve impulse transmission e.g. neuronal receptors
- Control of metabolism, growth and differentiation e.g. insulin, gene repressors, EGC
Give an example of a protein that consists of one polypeptide chain
Prolactin
What is the smallest functional protein?
A tripeptide called thyrotropin-releasing hormone
How many amino acids are there?
20
Describe the structure of an amino acid
Central carbon atom surrounded by 4 different groups: a hydrogen atom, an amino group (potentially basic), a carboxylic group (potentially acidic), and an R group
Which side chain gives each individual amino acid its chemical and biological properties
R group
Which groups on amino acids act as hydrogen acceptors and which act as hydrogen donors?
COOH (carboxyl group) = hydrogen donor
NH2 (amino group) = hydrogen acceptor
What are the 2 types of isomers that amino acids can be due to the 4 different groups attached to the central carbon?
D and L isomers
Which amino acid is the only one that does not have isomers?
Glycine
R group is H+
Which isomeric form of amino acids do proteins only contain?
L isomers
Where might you find D-isomers of amino acids?
Bacterial cell walls
Describe the ionisation of any given amino acid at pH1
Amino acid will be fully protonated. Cationic form (charge = +1)
Describe the ionisation of any given amino acid at pH 7.4
As pH becomes more alkaline, the first group to dissociate is the carboxylic acid group. Amino acid in zwitterion form (charge = 0)
Describe the ionisation of any given amino acid at pH 11
All protons lost: both carboxylic acid and amine group give up H+ (donating H+ to counteract rising pH). Amino acid is in anionic form (charge = -1)
How many groups of amino acids are there based on the chemical composition of the R group?
7
What are the 7 groups of amino acid based on the chemical composition of the R group?
- Aliphatic
- Hydroxyl
- Sulfar-containing
- Aromatic
- Acidic
- Basic
- Imino
Which amino acids are classed as aliphatic?
Glycine, alanine, valine, leucine, isoleucine
Which amino acids are classed as hydroxyl?
Serine, threonine
Which amino acids are classed as sulfur-containing?
Cysteine, methionine
Which amino acids are classed as aromatic?
Phenylalanine, tyrosine, tryptophan
Which amino acids are classed as acidic?
Aspartic acid, asparagine, glutamic acid, glutamine
Which amino acids are classed as basic?
Arginine, histidine, lysine
Which amino acids are classed as imino?
Proline
Which 9 amino acids are essential?
Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine
What does “essential” mean in terms of amino acids?
They cannot be made in the body - need to come from external sources e.g. diet
What is electronegativity?
- The power of an atom to attract electrons towards itself
- Results in uneven sharing of electrons within the covalent bonds in the R side chain
Which atom is the least electronegative?
H+
Left with a partial positive charge; other atoms will pull electrons away from it
Which amino acids can contribute to buffering?
Amino acids with polar R groups
Basic and acidic amino acids are polar
Which amino acids have side chains that are uncharged but polar due to partial charges in permanent dipole bonds in non-ionisable R groups?
Asparginine, glycine, serine, threonine, tyrosine, cysteine
How are amino acids joined together to form a polypeptide chain?
By a peptide bond
How is a peptide bond formed?
- Carboxylic group of one amino acid forms a peptide bond with the amino group of another amino acid
- Water is eliminated in the formation of the peptide bond
Why do R groups face on opposite sides/planes of the chain in large proteins?
R groups on opposite side of chain provides steric stability
Once proteins have been synthesised, some have post-translational modifications. What does this mean?
The R side chains of some amino acids are modified after the protein has been translated to give that protein a specific biological function
Which amino acid often has an extra hydroxyl group added to it as a post-translation modification?
Proline -> hydroxyproline
Which amino acid can have an extra carboxyl group added to it as a post-translational modification?
Glutamate -> gamma-carboxyglutamate
Which amino acid is phosphorylated as a post-translatonal modification?
Serine -> O-phosphoserine
Phosphorylation of which amino acid is the most common form of regulation of protein activity?
Serine -> O-phosphoserine
In hydroxyproline, what does the OH group do in collagen fibres?
OH group stabilies collagen fibre -> insufficient hydroxylation leads to scurvy
What can a lack of carboxylation of glutamate lead to?
Haemorrhage
Carboxylation of glutamate (forming gamma-carboxyglutamate) is important in prothrombin (required for clotting)
What is the primary structure of a protein?
Amino acid sequence
What is the secondary structure of a protein?
Arrangement in space of amino acids close to one another in a polypeptide chain e.g. alpha helix, beta pleated sheet
What is the tertiary structure of a protein?
- 3-D structure of all the atoms in a single polypeptide chain
- Interactions between R groups fold protein into compact 3-D shape
What is the quaternary structure of a protein?
- 3-D interaction of protein subunits in proteins with more than one polypeptide chain
