2 Protein Function Flashcards
Contrast the oxygen binding properties of haemoglobin and myoglobin and explain why haemoglobin is more suited to its role as oxygen transporter. 2.2
Haemoglobin is an allosteric protein that can exist in a low affinity state (T) and a high affinity state (R). The binding of oxygen makes haemoglobin into its R state. This means that haemoglobin shows a sigmoidal relationship between concentration of oxygen and affinity to oxygen.
Myoglobin’s affinity to oxygen increases with oxygen concentration hyperbolically.
Haemoglobin is a better carrier because more oxygen can be taken in the lungs (high affinity R state) and more oxygen can be delivered to the tissues (low affinity T state). With myoglobin this difference is less making it a less effective transporter.
Explain the physiological roles of myoglobin and haemoglobin 2.1
Haemoglobin and myoglobin transport oxygen from the lungs to the tissues.
Describe the major structural differences between oxygenated and deoxygenated haemoglobin and the molecular basis of cooperativity. 2.3
A haem group in a molecule of haemoglobin is connected via an Fe atom that is also bonded to four nitrogen atoms and it sits slightly below their plane. When O2 binds to this Fe, the Fe atom is slightly ‘pulled’ into the same plane as oxygen, this changes the overall shape of the haemoglobin and give it a higher affinity for oxygen.
Describe the effects of CO2, H+, 2’3-bisphosphoglycerate and carbon monoxide in the binding of oxygen by haemoglobin and the physiological significance of these effects. 2.4
BOHR effect: An increase of H+ (decrease in pH) or CO2 decreases haemoglobin’s affinity to oxygen. This is physiologically significant as it allows more oxygen to be released in metabolically active tissues where H+ and Co2 concs will be higher.
2’3-bisphosphoglycerate decreases haemoglobin’s affinity for oxygen this is physiologically significant as it allows the haemoglobin to be able to transport more oxygen to tissues (due to a bigger difference in affinities in the lungs and at tissues). This is particularly important at high altitudes where O2 concentration is lower.
CO binds to haemoglobin with a higher affinity to oxygen at the same sites, inhibiting oxygen from binding with haemoglobin. This makes CO a poison.
Appreciate that mutations in globin genes can give rise to diseases such as sickle cell anaemia or thalassemia. 2.5
Sickle cell anaemia- glutamine is change to valine in B globin (HbS). An area that was hydrophobic is now hydrophilic causing molecules to group and HbS to polymerise. These cells are more prone to lyse (anaemia) and more rigid which means they can cause blockages.
Thalassaemia is the name given to a group of genetic disorders where there is an imbalence of a and b globin chains (HbA =2a +2b). B thalassaemia means there is decrease B meaning a cannot form a stable tetramer. A thalassaemias have different levels of severity as there are multiple copies of a chains and b chains can form stable tetramers with increased affinity to oxygen.
Explain the effects of enzymes on chemical reactions 2.6
Enzymes increase the rate of reaction so the reaction reaches equilibrium faster by providing chemical pathways with lower activation enthalpies. Enzymes do not effect the position of equilibrium. Enzymes are not used up in the reaction. Enzymes are also high specific to 1 or few reactions.
Describe how reaction rates vary as a function of enzyme and substrate concentration. 2.7
Reaction rates increase hyperbolically in respect to substrate concentration and linearly with enzyme concentration. This is because doubling enzyme conc doubles the amount of reactions taking place however when substrate conc is doubled, enzymes are already partially saturated.
Define the terms activity, international unit of enzyme activity, Km and Vmax. 2.8
Activity - the rate at which enzymes break down substrate.
International unit of enzyme activity -
Km - the concentration of substrate at which V = 1/2 Vmax. Called the michaelis constant.
Vmax - the maximum velocity of a reaction: adding more substrate does not increase velocity.
Analyse and interpret kinetic data for enzyme catalysed reactions. 2.9
K
Describe the effects of enzyme inhibitors on enzyme kinetics and be able to distinguish between the two from simple graphs 2.10
Enzyme inhibitors decrease enzyme activity so that substrate is not broken down as quickly. There are two types of reversible enzyme inhibitors, competitive and non competitive.
Competitive inhibitors bind at the active site. They have no effect on Vmax (a very high concentration of substrate will ‘out compete’ the inhibitors) but increase the Km.
Non competitive inhibitors bind at a different sites to substrate. They lower the Vmax (same effect as decreasing enzyme concentration) but Km remains the same.
On a graph non competitive will not reach the same velocity but Km is in the same place. Competitive inhibitors shift the graph to the right as Km is at a higher substrate concentration.
Irreversible enzyme inhibitors bind covalently e.g. Nerve gases.
