#7 What I Learned Today

Biochemistry

Today in biochemistry, we lectured on the hexose monophosphate shunt (which is also known as the pentose phosphate pathway). The pathway is a major one in the metalbolic usage of glucose. The cell can use it for many processes including lipid synthesis, nucleotide synthesis (through the molecule ribose) and antioxidant synthesis. It does this mainly through the formation of NADPH, which is a molecule often used for many anabolic (creating) processes; NADP is often used in catabolic (destroying) processes.

The pathway takes 3 glucose-6-phosphate molecules and 6 NADP molecules and converts them to 2 glucose-6-phosphate, 3 carbon doxide, 1 glyceradehyde-3-P and 6 NADPH. This pathway occurs mainly in two distinct steps; however, the reaction is typically reversible, and thus can be used by a cell in a number of ways, depending on what molecules it needs at the time.

The first step is an oxidative phase, where glucose-6-P is oxidized first at Carbon-1, which converts the hydroxyl group into a ketone (gluconolcatone). This reduces an NADP to an NADPH. Water is then used to hydrolyze the bond between Carbon-1 and Oxygen to form an open-chained carboxylic acid (phosphogluconate). Next, the molecule is oxidized at Carbon-3 to produce another NADPH molecule, and an unstable keto-acid. This molecule is then stabilized by giving off a CO2 molecule, forming ribulose-5-phosphate (only 5 carbons!).

The second step is a rearrangement phase, where carbon is shuffled around in order to get as many 6-carbon fragments back as possible. At this point, ribulose-5-phosphate can take two different pathways, depending on the cell's needs:

(1) if ribose is NOT needed, the molecule flips it's hydroxyl group to form xylulose-5-P. It is then cut by transketolase where 2 carbons are removed from xylulose-5-phosphate, and added to ribose-5-P to form a 3-carbon and 7-carbon molecule. The 7-carbon molecule is then cut by transaldolase, which takes 3 carbons off and adds them to the 3-carbon sugar, forming a 4-carbon molecule (erytherose-4-P) and a 6-carbon molecule (fructose-6-P).

Fructose-6-P is essentially equivalent to glucose-6-P!

(2) if ribose is needed for nucleotide synthesis, the double-bonded O in ribulose is swapped from Carbon-2 to Carbon-1, which forms ribose-6-P.

This reaction is also very important in RBC as they form many free radicals due to having both iron and oxygen in their system. The cell converts these harmful radicals to H2O2 (peroxidase), and further use the enzyme glutathione reductase to break down the peroxidases before they damage the cell. NADPH is important in the formation of the GR enzyme. When deficient, precipitaiton of the RBC occurs, causing the cell membrane to become rigid, and thus the macrophages attack the RBC, causing acute hemolysis.

Principle of Dental Materials

We discussed the properties of two common hydrocollodial dental materials: alginate and gypsum. Alginate is the material used to take impressions of the mouth, which form an inverted mold. Gypsum is the plaster material that is poured into the alginate impression in order to form an actual model of the patient's mouth. The model can then be used in diagnosis and in crown preparation.

Alginate is derived from seaweed, and is made up of a linear polymer of anhydro-beta-D-mannuronic acid. Algiante reacts with hydrated calcium sulfate and water to form calcium alginate and other products. This is a non-reversible reaction which sets in about 1-5 minutes, depending on the type of alginate used. Alginate is very cheap, is flexible, easy to use and can reproduce oral detail to an acceptable level. There are some rubber-based impression materials that can capture more detail, but they tend to be of higher price. When taking alginate impressions, it is important to take into account some important things which may affect the properties of alginate, including the amount of water mixed (too much is bad!), rate of impression removal from mouth, storage time, and saliva and blood contaminations.

For the gypsum reaction, calcium sulfate hemihydrate (half a mol of water) reacts with water to form gypsum. Gypsum comes in many different types, depending on the model's use. These include model plaster for study models, and dental stone for lab cases. When adding water, one must be careful not to add too much as excess water causes porosity in the plaster, ultimately lowering its strength. One must be careful as the gypsum has two separate setting phases. The initial set takes between 8-16 minutes and is still very weak and fractures easily. After 20-60 minutes, the set stone is much harder and at this point is ready to be separated from the impression.

Dental Anatomy

Our lecture today focused on the permanent maxillary and manibular canines.

The canines are considered to be the most important tooth in the dentition. Not only is it important in tearing food, but it also acts to support the facial muscles and is one of the strongest tooth in the mouth due to having the longest root in the mouth. The shape of the labial surface is that of a pentagon, with a cusp tip that is centered over the body of the crown. This tip results in two incisal cusp ridges, with the distoincisal ridge being longer than the mesioincisal ridge. The resulting mesial outline is slightly convex, while the distal outline is slightly concave With wear, the cusp tip moves distally.

The height of contour in the mesial is found on the junction of the incisal and middle third, while the height of contour in the distal is found in the middle third. Two ridges are found on the labial and lingual surfaces, forming mesiolabial and distolabial developmental depressions on the front, and mesiolingual and distolingual fossa on the back. The lingual of the tooth also has a cingulum (offset to distal), mesial and distal marginal ridges; all of which are very prominent on the maxillary. The distal marginal ridge is often more irregular than the mesial margin.

From the incisal view, the shape is seen as an asymmetrical diamond, with the mesial half being more bulky than the distal. The distal also sees a distolingual concavity on the maxillary.

The root often has two concavities on both the distal and mesial, with the distal being more prominent.

The mandibular differs from the maxillary in that the crown is actually longer incisogingivally, but smaller on both the mesial-distal and facial-lingual dimensions. As well, the distoincisal line angle is more rounded. This results in the mesial outline to be a bit longer and straighter than the distal.