Oxygen Saturation Notes PDF

**[Measuring O2 content in blood]** - The arterial oxygen saturation is expressed as a percentage (represents the overall % of Hb binding sites occupied by oxygen). - In healthy individuals at sea level SaO2 is 96 -- 98% - The maximum amount of O2 the blood can carry when fully saturate...

**[Measuring O2 content in blood]** - The arterial oxygen saturation is expressed as a percentage (represents the overall % of Hb binding sites occupied by oxygen). - In healthy individuals at sea level SaO2 is 96 -- 98% - The maximum amount of O2 the blood can carry when fully saturated is termed the O2 carrying capacity, which, with a normal haemoglobin concentration is around 20ml oxygen per 100 ml blood. - Measuring the amount of O2 is performed in 2 ways: Pulse oximeter (less accurate) measures the pO2 in arterial blood and gives an indication of how much O2 is present in arterial blood. Unable to differentiate dyshaemoglobin (COHb/MetHb) - CO oximeter -- (more accurate) measures of SaO2 in arterial blood -- able to differentiate oxyHb and dyshaemoglobins (COHb/MetHb). Best for critical care. **Protein Structure (a hierarchy)** - Proteins are composed of a sequence of amino acids linked by peptide bonds (primary structure) - Short sequences of amino acids form into stable elements such as A-helics, b-sheets, b-turns (secondary structure) - A single polypeptide chain of a globular proteins (myoglobin) folds up into 3D shape (tertiary structure) - For multi-subunit proteins (e.g. haemoglobin) each subunit associates with its neighbours through many non-covalent forces to form a stable complex (quaternary structure) A protein's surface reflects its environment: Aqueous (e.g., blood) -- protein surface consists of polar amino acids. Lipid (non-polar) environments (e.g. membrane proteins) -- protein surface consists of non-polar amino acids. **Myoglobin -- O2 storage** - Found primary in muscle tissue - Single polypeptide chain - Gloubular protein with 8 a-helical segments, joined by bends - Contains one haeme prosthetic group, binds one O2 molecule Myoglobin has a high affinity for O2 which makes it a good storage molecule. Myoglobin has a hyperbolic shaped O2 dissociation curve. **What is an Oxygen dissociation Curve?** - A graph showing the relationship between the amount of O2 (saturation) bound to Mb or Hb versus pO2. - Below is the ODC for myoglobin -- Mb is almost completely saturated at high pO2 which stays bound unless the pO2 gets very low. - This is great for storage protein. Notice: The hyperbolic shape. Mb has high affinity for O2. **Mb has high affinity for O2 and a low KD** Reversible binding of O2 to Mb is described by the simple equilibrium: Protein + ligand [\$\\overset{\\leftarrow}{\\text{Kd}}\$]{.math.inline} protein -- ligand complex KD (dissociation constant) is a measure of affinity of a protein to its ligand. Note the inverse relationship (i.e. small KD reflects a high affinity) Mb has a high affinity for O2 so low KD KD = pO2 at which 50% the binding sites are filled with O2. **Haemoglobin (Hb) -- O2 transport** Hb has 4 subunits (tetramers), each of which is very similar in 3D to b-subunit of MB. Hb has 2 identical a-subunits and 2 identical b-subunits (a2b2) a-subunits is 141 amino acid long, b-subunit is 146 amino acid long. 1 haeme and 1Fe2+/subunit Hb binds up to 4 O2 molecules. Hb is better at transporting O2 and is designed to be sensitive to small changes in the pO2. **Structure of Haeme** Haeme is composed of a porphyrin ring covalently bound in a deep pocket in each Hb subunit (protect the Fe2+) Fe2+ ion in centre. Fe2+ makes 6 covalent bonds. 4 to the planar haeme ring, one to the histidine amino acid of Hb and one to O2. Haeme has a concave shape in deoxy (T) state, whereas, it has a flat, planar shape in oxy (R) state. **Colour of oxygenated and deoxygenated haemoglobin in blood** - When O2 binds the electronic configuration in haeme complex changes turning from darker red (deoxy Hb) to scarlet red (OXYHb) - This underpins how the pulse oximeter works. - But also reflects molecular changes in Hb that affect O2 binding. **Key principles of proteins structure and function** - Reversible, non-covalent binding of small molecules (ligands) to proteins occurs at sites on the protein called binding sites. The binding site is complementary to the ligand in size, shape, charge and polarity (specificity) - Proteins and ligands are mobile (not rigid) and so we can wiggle around for best fit. - Ligand binding is accompanied by a conformational change in the protein (&/or ligand) which can facilitate and impact on its function. E.g. binding of H+ to haemoglobin causes a conformational change leading to off-loading of O2 tissues. - Proteins with high affinity for a ligand bind it more tightly and don't release it as easily. - Mb has high affinity for oxygen - DexoyHb has low affinity for O2 whereas OxyHb has a high affinity for oxygen. Question: how can the same protein have different affinities for the same ligand? **Cooperative Binding in Hb changes its affinity towards O2.** - Hb responds to its environments such as changes in partial pressure of O2. In the lungs, where pO2 is high, the binding of O2 to one subunit of deoxyHb can increase the affinity of neighbouring subunits to O2, effectively making it easier for O2 bind. - This enhances the transition to oxyHb. This is called cooperative binding. - Take home messages: Hb can change its affinity for O2 through cooperative binding, from high affinity (in the lungs) to low affinity (at the tissue) which means It can easily pick up or release O2 respectively in the appropriate environment. - Whereas Mb, being only one single subunit protein, only has one affinity for O2 that is high affinity. **Transition of DeoxyHb to OxyHb** - DeoxyHb is in the T (Tense) form with a low affinity for O2. The subunits are stabilised by ion pairs. - Once 2 subunits, in the T state, bind O2 a T-R transition occurs causing a change to the R (relaxed) state. This makes it easier for the remaining subunits to bind O2. - The main difference is in the alignment of alpha and beta subunits which slide past one another. As they do a number of ionic bonds are broken. - This is called cooperative binding. **Interpreting Hb's oxygen dissociation curve** - Unlike Mb, Hb can change its affinity for O2 from high to low affinity (this is characterised by a sigmoidal shaped curve). - This is achieved by responding to changes in the environment. E.g. pH, CO2, Temp, 2,3-BPG - HPG can remain highly saturated even with large decrease in pO2. - O2 rapidly combines with Hb as the pO2 increases (& rapidly dissociates as pO2 decreases). - High affinity in the lungs (OxyHb/R state) to load up O2. - Low affinity in the tissues (deoxyHb/T state) to offload O2. S shaped curves can only be produced by proteins with more than one subunit. **Binding of small molecules to Hb changes its affinity toward O2** - As we've seen partial pressure O2 changes the affinity of Hb towards O2. A number of other factors influence the affinity of Hb for O2. - Haemoglobin can bind other small molecules at sites away from the O2 binding site. - These molecules are called allosteric effectors. They include: H+ (increased acid load/decreased pH) CO2 (reflecting an increased acid load, volatile acid) forming carbaminoHb 2,3 -- bisphosphoglycerate (2,3 -- BPG) - Their binding causes a conformational change in Hb structure which leads to a changed affinity for O2 and altered functionality. This is called allosteric regulation. - The binding of H+, CO2 and 2,3 -- BPG stabilise the T state (deoxyHb) which is convenient since levels of these molecules are higher in the periphery thus facilitating O2 off-loading where its needed.