Proteins Flashcards
What are collagen fibres
Main component of connective tissues and found in skin, tendons, organs and bone
Function of proteins
Provide structure, transport molecules, defence, biological catalysts and regulation of genes
What is haemoglobin
Selective delivery of O2 to metabolic tissues. It has 4 protein subunits per molecule. Each subunit contains a haem group that can bind 1 oxygen molecule
What is low density lipoprotein
It is composed of a phospholipid shell and a single molecule of apolipoprotein B
What is the function of LDL
It is used to transport cholesterol between cells via the circulatory system
How is the uptake of LDL particles mediated
By the LDL receptor that binds LDL and facilitates internalisation
What is the structure of an antibody
2 identical heavy chains and 2 identical light chains covalently linked by disulphide bonds
What is lysozyme
It catalyses the cutting of polysaccharide chains. It binds to the polysaccharide chain, catalyses the cleavage of a specific covalent bond and releases the cleaved products. It remains unchanged at the end of the reaction
What is the function of the lac repressor
It controls production of proteins metabolising lactose in bacteria
How does the lac repressor work
It binds to DNA and prevents expression of the gene in the absence of lactose
What is pKa
It is the pH at which dissociation is 50% complete
What influences the charge of an amino acid
pH
What does dissociation largely occur over
2 pH units centred on the pKa
What is receptor mediated endocytosis
- A reduction in pH in the endosome causes a change in conformation of the LDL receptor due to the presence of histidine residues within the protein
- LDL can no longer bind and is released to the lysosome
- Patients with familial hypercholesterolemia frequently have mutations in the histidine residues of the LDL receptor
What is a peptide bond
It is a covalent bond formed when the carbon from the carboxylate group shares electrons with the nitrogen atom from the amino group of a second amino acid
What are the constraints on a peptide bond
- It doesn’t permit rotation
- Bulky R-groups are positioned on either side of the backbone which limits the no. of 3D conformations possible for a polypeptide
What is secondary structure
It is the initial folding patter of the linear polypeptide. It is stabilised by hydrogen bonds
Describe the alpha helix
It is right handed and each turn has 3.6 amino acid residues. It is stabilised by H bonds between amino and carboxyl groups of every 4th amino acid
Describe beta sheets
Extended stretches of 5 or more amino acids are called beta strands. They are organised next to each other to make beta sheets. H bonding pattern varies depending on type of sheet
Describe the bend/loop structure
Polypeptide chains can fold upon themselves forming a bend or a loop. Usually 4 amino acids are required to form the turn
Describe tertiary structure
It is the 3D folding of secondary structure
What is a disulfide bond
The SH groups of 2 neighbouring cysteine residues form a covalent -S-S- bond
Describe quaternary structure
It is the association of more than one polypeptide. The protein can be called an oligomeric protein (made up of multiple subunits). Disulfide bonds stabilise the oligomeric structure
What is the structure of haemoglobin
It is a symmetrical assembly of 2 different subunits (2 alpha globin and 2 beta chains). Each of the polypeptide chains contains a haem molecule, which binds O2
What causes sickle cell anaemia
- A single amino acid change at position 6 in the beta chain of haemoglobin
- Hydrophilic glutamic acid to hydrophobic valine
- This causes sickling of erythrocytes due to aggregation of mutated haemoglobin that forms stiff fibres (change in surface chemistry of the protein)
What are the effects of pH and CO2 conc on haemoglobin
- O2 binding occurs with higher affinity at high pH (lungs) and lower affinity at low pH (peripheral tissues)
- O2 loading is therefore easier in the lung and unloading/delivery occurs much faster where O2 needed
- CO2 (acidic) builds up during exercise, which lowers blood pH facilitating faster oxygen delivery
What is tropocollagen
It is the building block of the collagen fibre and consists of 3 polypeptide chains with a left handed twist wound together in a right handed supercoil
What is the significance of glycine
It is vital for the formation of the tropocollagen triple helix as it has a small side chain that allows tight turns
What is the significance of proline
It imposes left hand twists in the helix that provides main stabilising force of the structure
What is hydroxyproline
It is formed when proline becomes hydroxylated. It forms strong H bonds that help stabilise the triple helix
