Biochem Exam II Flashcards
How is the Heme prosthetic group stabilized in myoglobin?
- A Fe(II) molecule is stabilized within a heterocyclic ring structure of 4 pyrrole groups connected by methene bridges
- The Fe(II) molecule is coordinated by the 4 porphyrin N atoms and an N atom of His F8, the proximal histidine
- O2 binds to Fe(II) at the 6th ligand position and His E7, the distal histidine, binds to the O2
- 2 hydrophobic side chains ValE11 (left) and Phe CD1 (right) give additional structural stability
What happens to the myoglobin molecule when meat spoils?
The helices that stabilize the heme structure begin to degrade which allows the hydrophobic binding pocket to get disrupted. This means the ferris (Fe(II)) ion is able to be oxidized into a ferric (Fe(III)) ion meaning O2 reacts with Fe(II) and does not bind to the pocket containing iron, which causes the meat to turn grayish in color. Heat (cooking the meat) speeds up this process. When the heme is disrupted, Fe(II) gets oxidized meaning oxygen does not bind to the hydrophobic binding pocket and myoglobin cannot function.
What is the function of myoglobin?
Myoglobin facilitates O2 diffusion in muscle. This is slow due to low O2 solubility. Myoglobin also has the effect of storing O2 within muscles.
How do the subunits of hemoglobin interact?
-a1-B1 and a2-B2 interface at 35 residue contacts
- a1-B2 and a2-B1 interface at 19 residue contacts
What are the structural differences between oxyhemoglobin (R state) and deoxyhemoglobin (T state)?
In the presence of oxygen one aB dimer is rotated ~15 deg closer to the other aB dimer along a 360 deg axis
What happens when hemoglobin is oxygenated?
When oxygen binds to hemoglobin, the Fe(II) atom is pulled 0.4 angstroms into the heme plane shifting the alpha helix F, which has the effect of shifting the helix into a tighter conformation. When this occurs an R state is formed from the deoxygenated T state.
What do the oxygen binding curves for myoglobin and hemoglobin suggest about their functions?
In the lungs the O2 partial pressure is 100 torr and in the tissues the O2 partial pressure is 20 torr. As myoglobin moves from the lungs to the tissues, even as the O2 partial pressure greatly decreases, the fractional saturation of myoglobin only decreases by around 7%, making it a poor oxygen depositor. This suggests that myoglobin functions to retain O2 intracellularly for storage rather than deposit it to tissues.
In contrast, as hemoglobin moves from the lungs to the tissues, where there is a lower O2 partial pressure, the fractional saturation of hemoglobin decreases by 66%, meaning hemoglobin is much more likely to give up its oxygen as it goes from a high to a low partial pressure of oxygen than myoglobin. This suggests that hemoglobin functions as an oxygen depositor to tissues.
How can the p50 illustrate that hemoglobin is able to more readily deliver oxygen to tissues than myoglobin?
The p50 refers to the partial pressure of O2 that is required to give 50% oxygen binding saturation. The p50 for myoglobin is 2.8 torr and the p50 for hemoglobin is 26 torr. Since we know the partial pressure of oxygen at tissues is 20 torr, the p50 can give an estimate of the saturation for myoglobin and hemoglobin at tissues. We know hemoglobin has an oxygen saturation of lower than 50% and myoglobin has an oxygen saturation of much higher than 50% at tissues, and a lower oxygen binding saturation for Hb suggests Hb is better able to readily deliver O2 than Mb.
How does the decrease of blood pH at tissue capillaries allow for the release of more O2 from hemoglobin (The Bohr Effect)?
When the blood pH decreases, this means excess protons are present at tissue capillaries. This results in increased oxygen deposition since when gas exchange occurs, Hemoglobin takes up the excess H+, and the H+ forms more ionic interactions that stabilize the deoxygenated T state rather than the oxygenated R state, which results in more O2 being released.
This means that the fractional saturation of O2 is decreased when pO2 is 20 torr at tissue capillaries, and this has the effect of shifting the binding curve to the right. A 0.2 decrease in blood pH generally results in a ~10% increase of the difference in fractional saturation between the lungs and tissues, resulting in ~10% more oxygen deposition.
What are three different ways the Bohr effect is used to promote oxygen release in hemoglobin?
- a red blood cell enzyme carbonic anhydrase catalyzes the production of bicarbonate and protons and decreases blood pH at tissue capillaries
H2O + CO2 –> H+ + HCO3- - the formation of lactic acid also decreases blood pH at tissue capillaries, generally in active muscles (~10% more O2 deposition)
- carbamate is formed as CO2 combines with the N-terminal of amino acids and this reaction produces protons which increase blood pH
How can D-2,3-biphosphoglycerate control oxygen binding/deposition?
D-2,3-biphosphoglycerate is a highly negatively charged molecule that binds to a positively charged, allosteric site within hemoglobin and stabilizes the T state.
The presence of BPG shifts the O2 fractional saturation curve to the right (higher pO2) releasing more O2. The absence of BPG shifts the curve to the left releasing less O2.
Why does O2 flow from maternal oxyhemoglobin to fetal deoxyhemoglobin?
BPG binds tighter to adult hemoglobin than fetal hemoglobin meaning adult Hb has a lower affinity for O2 and fetal Hb has a higher affinity. Since BPG concentration is the same in fetal and maternal circulation, this results in O2 moving from maternal red blood cells to fetal red blood cells. Fractional saturation is higher in fetal red blood cells at the same pO2 than maternal red blood cells enabling this flow.
What digestive enzymes are present in the mouth and what are their functions?
Salivary a-amylase hydrolyzes a 1-4 bonds that form polysaccharide chains in sugars but not a 1-6 bonds that link branching chains together. This results in broken down sugars being formed.
Lingual lipase begins to hydrolyze triglycerides (triacylglycerol) into diacylglycerol and monoacylglycerol.
What digestive enzymes are generally found in the stomach and how do they function?
Pepsin hydrolyzes proteins after an acidic pH of 1-2 in the stomach denatures the food.
What digestive molecules are commonly found in the small intestine?
Oligopeptides and acidic digestive production from the stomach stimulate the production of secretin and cholecystokinin (CCK) from the small intestine respectively.
Secretin stimulates sodium bicarbonate (NaHCO3) secretion from the pancreas to neutralize the acidic contents of the stomach
After the contents are neutralized, CCK stimulates the pancreas to release numerous enzymes and stimulates the gallbladder to release bile salts, which aid in fatty acid digestion.
How are zymogens different from enzymes?
Digestive enzymes tend to be secreted as zymogens (also known as inactive precursors or proenzymes) that then get activated into enzymes
How is pepsinogen activated into pepsin?
Pepsinogen has a low level of enzyme activity that allows it to activate itself to some capacity in an acidic environment such as the stomach. The active pepsin enzymes can then activate the remaining pepsinogen zymogens.
How is trypsinogen activated into trypsin? What is the function of trypsin?
Enteropeptidase is secreted by epithelial cells as a functional enzyme and activates trypsinogen into trypsin. Trypsin can then further activate the remaining pancreating zymogens such as elastase, carboxy-peptidase, chymotrypsin, lipase, and can further activate trypsinogen into trypsin.
What are the functions of the pancreatic enzymes?
Lipase - further breaks down fatty acid chains, removes them one at a time from the triglyceride
Chymotrypsin - cleaves the peptide bonds of aromatic amino acids
Trypsin - cleaves the peptide bonds of basic amino acids
Carboxypeptidase - removes amino acids one at a time from the C terminus of an amino acid chain
Elastase - general protease that breaks down proteins
What role does peptidase play in the breakdown of amino acids?
Peptidases complete the breakdown of oligopeptides into singular amino acids, tripeptides, and dipeptides until only amino acids are present that can be transported through the bloodstream
What enzymes digest sugars in the small intestine?
Pancreatic a-amylase hydrolyzes a-1-4 bonds in polysaccharides to produce maltotriose, maltose, and a-limit dextrin
These enzymes are sugar enzymes that are located on the epithelial cell surface
Maltase - converts maltose into glucose
a-glucosidase - digests maltotriose and other oligosaccharides
a-dextrinase - digests the limit-dextrin
sucrase - digests sucrose to glucose and fructose
lactase - digests lactose to glucose and galactose
