Amino Acids & Polypeptides
Proteins
- Proteins are polymers (and macromolecules) made of monomers called amino acids
- The sequence, type and number of the amino acids within a protein determines its shape and therefore its function
- Proteins are extremely important in cells because they form all of the following:
- Enzymes
- Cell membrane proteins (eg. carrier)
- Hormones
- Immunoproteins (eg. immunoglobulins)
- Transport proteins (eg. haemoglobin)
- Structural proteins (eg. keratin, collagen)
- Contractile proteins (eg. myosin)
- Because all genes code for proteins, all of the reactions necessary for life are dependent on the function of proteins
Amino acids
- Amino acids are the monomers of polypeptides
- There are 20 amino acids found in polypeptides common to all living organisms
- The general structure of all amino acids is a central carbon atom bonded to:
- An amine group -NH2
- A carboxylic acid group -COOH
- A hydrogen atom
- An R group (which is how each amino acid differs and why amino acid properties differ e.g. whether they are acidic or basic or whether they are polar or non-polar)
- The R group can be as simple as another hydrogen atom (glycine), right through to complex aromatic ring structures (eg. phenylalanine)
The generalised structure of an amino acid
Peptide bond
- In order to form a peptide bond a hydroxyl group (-OH) is lost from the carboxylic group (-COOH) of one amino acid and a hydrogen atom is lost from the amine group (-NH2) of another amino acid
- The remaining carbon atom (with the double-bonded oxygen) from the first amino acid bonds to the nitrogen atom of the second amino acid
- This is a condensation reaction so water is released
- Dipeptides are formed by the condensation of two amino acids
- Polypeptides are formed by the condensation of many (3 or more) amino acids
- A protein may have only one polypeptide chain or it may have multiple chains interacting with each other
- During hydrolysis reactions, the addition of water breaks the peptide bonds resulting in polypeptides being broken down into amino acids
Amino acids are bonded together by covalent peptide bonds to form a dipeptide in a condensation reaction
Examiner Tip
You will be expected to recognise whether an unfamiliar molecule is an amino acid or polypeptide so look for the functional groups (amine and carboxyl). When asked to identify the location of the peptide bond, look for where nitrogen is bonded to a carbon that has a double bond with an oxygen atom, note the R group is not involved in the formation of a peptide bond.