Proteins Haileybury Astana IB SL Biology

Proteins are made up of amino acids +ve -ve The R group is different for each of twenty amino acids. Here, the structure is shown in its ionised form!

More usually shown as: The acid group is a carboxylic acid group as found in fatty acids.

Amino acids are assembled into polypeptides and proteins

Exam version: Amino acid 1 Amino acid 2 hydrolysis condensation Peptide bond This happens repeatedly during TRANSLATION in the ROUGH ENDOPLASMIC RETICULUM in EUKARYOTES to produce POLYPEPTIDES and PROTEINS

Some amino acids are hydophobic and the R-groups will face inwards. Some have R-groups that are hydrophilic and face outwards. Some will for hydrogen bonds and create -pleated sheets or alpha helixes. Disulphide bridges (covalent R-groups Changing pH will change the orientation of some of the R-groups causing the three-dimensional shape of a protein to change (denature) – this can be reversed by restoring the pH.

Primary Structure The primary structure of a protein is the order of the amino acids. The only bonds recognised at this level of protein structure are the covalent (peptide) bonds between the amino acids.

Secondary Structure An -helix structure is found in structural proteins such as hair (keratin) as well as some enzymes and antibodies -pleated sheets are found in some structural proteins, especially in those whose function requires strength such as silk.

Tertiary Structure Tertiary structure refers to the way in which proteins folds due to disulphide bridges (covalent), ionic bonds, hydrogen bonds and hydrophobic interactions. Oxytocin Disulphide bridges between cysteines

β – pleated sheet α - helix

Quaternary Structure The quaternary structure is where the protein consists of more than one polypeptide chain such as haemoglobin shown here. Haemoglobin consists of four polypeptides as well as four ‘prosthetic’ haem groups (US heme)

Examples of proteins and their functions Rubisco An enzyme that controls the carbon fixing reaction of photosynthesis. Insulin A hormone that controls blood sugar. Immunoglobulins = Antibodies – recognise antigens on pathogens. Rhodopsin Pigment found in the retina. Spider silk Spun by spiders to make webs. Collagen Collagen is the main structural protein in the extracellular space in the various connective tissues in animal bodies. That is, it holds tissues together. Keratin Forms hair and nails Haemoglobin Transports oxygen