number 19 Done by حسام ابو عوض Corrected by وسيم ابو عبيدة Doctor د.نايف 1 P a g e
GAGs and Glycoproteins: GAGs: long, unbranched heteropolysaccharides, made from زunits repeating disaccharide [Acidic sugar Amino sugar]n. n=1,2,3. Amino sugar: is a sugar molecule in which a hydroxyl group has been replaced with an amine group. The amino sugars are usually acetylated (adding an acetyl group to the amine group), it may also be sulphated on carbon 4 or 6 or on a nonacetylated nitrogen. The amino sugar in GAGs is either glucosamine or galactosamine. Acidic sugar: monosaccharides with a carboxyl group. The acidic sugar in GAGs is either glucuronic acid or its c5 epimer, Iduronic acid. Glycoproteins: Made by combining a glucose with an amino acid. This can happen by an O- glycosidic bond (UDP-glucose reacts with threonine, serine or lysine via their OH) or an N-glycosidic bond (UDP-glucose reacts with asparagine via its amide). Glucuronate Pathway: Glucuronic acid can be obtained in small amounts from the diet or degradation of GAGs. Glucuronic acid is an essential component of GAGs and it is required for detoxification reactions of a number of insoluble compounds, such as bilirubin, steroids and several drugs. The active form of glucuronic acid that donates the sugar in GAGs and otherglucuronylating reactions is UDP-glucuronic acid which is produced by oxidation of UDP-glucose. 2 P a g e
UDP-glucose is converted to UDP-glucuronate using the UDP-glucose dehydrogenase enzyme, NADH + H+ are produced in the process. The glucuronate from the UDPglucuronate can be donated to many substances and toxins (UDP-glucuronate transferase enzyme is needed) solubilising 3 P a g e
them and allowing them to be excreted with urine [E.g. bilirubin is converted to bilirubin diglucuronide, which is soluble and can be excreted. NB: this example is important]. The glucuronate needs to be converted to its UDP form to be activated (like all other sugars). The UDP-glucose participates in many different metabolic pathways as shown:udp-glucose is used in the production of proteoglycans (in addition to the glycoproteins). Antioxidants (From ROS chapter in Mark s book): Oxygen Toxicity Oxygen is a biradical, a molecule that has two unpaired electrons in separate orbitals. The oxygen superoxide and the hydroxyl radical are free radicals, while the H₂O₂despite being a ROS, is not a free radical (does not contain free unpaired electron). 4 P a g e
90% of the O₂ we get is used in the aerobic respiration. The other 10% is divided: 5-7% is used in other oxidation-reduction reactions, the oxygenase enzymes (monooxygenases and dioxygenases, refer to the previous sheet). 3-5% is used in the production of ROS from oxygen, this amount is useful for protective mechanism in the body against many microbes, but this amount could increase in some inflammatory responses and harm the body. Examples of ROS: OCL, H₂O₂, OH and the superoxide free radical (O₂ ) The ROS can cause atherosclerosis, respiratory diseases, Parkinson s disease, cancer, diabetes, liver damage, motor neuron damage and appear to have a role in aging. The ROS can be behind complications from a disease. (The WBC s produce them to destroy some pathogens but then the ROS themselves can cause damage). They can damage proteins, lipids, nucleic acids and carbohydrates. The structures most prone to damage are the ones containing proline, histidine, arginine, cysteine and methionine. Oxygen in complex 4 of the oxidative phosphorylation, in the process to produce water, produces many intermediates that are actually ROS. Side note: electrons cannot escape from complex 4 because they are well contained by the electron acceptors there and the iron-sulphur centres. Fenton Reaction H₂O₂ is changed to OH and OH (most potent ROS). This reaction occurs through oxidising Fe²+ (or Cu+) to Feᵌ+ (or Cu²+). (This reaction is very important). Any free iron can cause problems because of this reaction (free radical is produced) so it is well secured by the body (ferritin stores iron, if iron amountsexceedferritin s storage capacity it changes to hemosiderin to be able to store more), but if its intake is too high then there will be some free iron [high iron in the blood is often seen with blood transfusion as iron is preserved in the blood so the patients are given iron chelators (make iron soluble)]. Side Note: Most young females have lower iron intake than needed while young males often have what is enough for them. Females need iron more than males. 5 P a g e
Haber-Weiss Reaction O₂ reacts with H₂O₂ (also H+ is involved in the process) to produce O₂, H₂O and OH. ROS and Lipids The ROS can damage membranes and organelles. This means that there will be gaps in the membrane (lipids are peroxidised, mainly poly-unsaturated ones. Proteins and DNA are also badly affected) allowing ions and water to enter causing cellular swelling. Following we go through the reaction of the ROS with the Poly-unsaturated Fatty-acids (PUFA). So, as we see a chain reaction begins in which a new free radical is produced each time. The LOOH (lipidhydro-peroxide) produced by the reaction above is degraded to Malondialdehyde (MDA) and a lipid peroxide. The MDA goes to the blood then is excreted with urine. The MDA concentration/amount in the urine is used in the laboratory as a marker to predict the amount of ROS present in the body in many chronic disease. (Other markers from other molecules are present, but this is the one tested for most often). There are some defensive mechanisms against ROS in the body like the SOD (Superoxide-dismutase) which converts the superoxide to hydrogen-peroxide 6 P a g e
which is then converted by catalase (only for inorganic hydrogen peroxide) to water and oxygen or reduced to water by the oxidation of glutathione (glutathione peroxidase enzyme is used). For glutathione to be reduced back (so that it can be used again) the enzyme glutathione reductase (which needs NADPH to function) is required. Other endogenous antioxidants include: melatonin, bilirubin, lipoic acid and ubiquinone. (They are sold as supplements in pharmacies, using these supplements is advised for elderly because they have a higher concentration of ROS in their bodies). Also, it is important to get some anti-oxidants from your diet (vitamin E, vitamin C and β-carotenes). In addition, there are some repair mechanisms in the body for the damaged substances (DNA, amino acids and fatty acids). An extremely important point to note is that our body doesn t let itself prone to attacks and damages easily. Wherever the ROS are needed you would notice that they are stored in their own vesicles (peroxisomes) with many antioxidants available for use within that cell (this is called compartmentation). This is often seen in the liver, adrenal glands and kidneys. Other Dietary Antioxidants Flavonoids These are polyphenol compounds, which contain many phenol rings in their structures like the molecule quercetin. You can obtain these flavonoids from many different dietary sources like green tea and chocolate. These flavonoids have the ability to reduce the production of ROS like superoxide ion by inhibiting the xanthine oxidase enzyme (this enzyme produces ROS). They also act as free radical scavengers (they react with the free radical ending its problem). They have the ability to maintain vitamin E and can chelate (a specific chemical process) iron and copper. [Extra: some types of flavonoids (between brackets is a source of that flavonoid type): Catechins (strawberries), Kaempferol (apples), quercetin (beans) and epicatechins (cocoa)]. 7 P a g e
Vitamin Antioxidants Vitamin E: its only physiological job in the body is to remove the free radical state from substances. It does this by donating an electron to a free radical molecule (no longer becomes a radical) (vitamin E becomes a radical) then reacting with another free radical (both radicals react, vitamin E and the 2 nd free radical molecule) neutralising its effect.the molecule α-tocopherol is vitamin E. The reaction is shown in this diagram: Vitamin C: it can accept an electron from the superoxide ion, hydrogen peroxide, HOCL and the lipid peroxyl radicals. It can also return the reduced vitamin E to its active form. Carotenoids: these also accept an electron from the lipid peroxyl radicals. The diagram shows the action of vitamin C and carotenoids: (The doctor never mentioned the details seen in this image, but they are in the slides, so they were added to be on the safe side). 8 P a g e