#22 What I Learned Today

Biochemistry

Today in Biochemistry, we looked at regulation methods for the citric acid cycle. The cycle can be regulated a number of ways.

The first method uses the products of the cycle to perform allosteric inhibition. Two of the products of the reaction, NADH and acetyl-CoA, inhibit the initial step where pyruvate is converted to acetyl-CoA by pyruvate dehydrogenase.

The cycle can also be covalently regulated by the addition or removal of a phosphate group. When phosphorylated, the enzyme is inactivated. When dephosphorylated, the enzyme is activated. With that in mind, two products of CAC, NADH and acetyl-CoA, activate the activity of protein kinase. Thus, the formation of lots of product results in the shutting down of the cycle. Protein kinase is inhibited by high levels of pyruvate and ADP. Thus, lots of reactants leads to the subsequent production of ATP through the running of Kreb's cycle, and ultimately the electron transport chain.

Finally, the citric acid cycle can be regulated by controlling the activity of enzymes in irreversible steps of CAC. This includes the steps with the following enzymes:

1. citrate synthase
2. isocitrate dehydrogenase
3. alpha-ketoglutarate dehydrogenase

Deficiencies in pyruvate dehydrogenase complexes can have devastating results as it becomes difficult to get energy from pyruvate and glucose. It tends to affect neural tissues since the brain is essentially only able to use glucose for energy.

Principles of Dental Materials

In dental materials, we began to look at the composition and structure of dental amalgam. This lecture is a re-hash of what we learned in Operative class last week. To summarize, amalgam is a setting procedure where amalgam alloy (powder) reacts with mercury (liquid) to form set amalgam (solid).

Advantages of amalgam include:

• high durability (lasts longer than most direct restorative materials)
• setting material (allow for easier placement by the dentist)
• good consistency (easy placement and shaping)
• low cost

Disadvantages of amalgam include:

• gray-metallic luster (unnatural appearance)
• contains mercury (very small amount released during chewing; greatest exposure comes from the setting and removal of the amalgam)
• low tensile, ductility and toughness (brittle alloy that cannot be used in thin sections)
• setting reaction takes time (strength builds over 24 hours, patient needs to be careful about mastication initially)
• Creep, dimensional change (can lead to marginal breakdown, but vary rarely seen in new copper amalgams)
• tarnish (the darkening of metal in oral cavity. Rarely seen in newer amalgam, but some patient's saliva content may still cause it)
• corrosion (due to gamma-2 phase, but rarely seen in newer amalgam)

Dental Anatomy

We had a practical examination today, which consisted of stations of Fuller & Denehy drawings and Kilgore teeth. Some of the questions asked for tooth identification, while others asked for specific tooth structures. It's important to know 'standard' (understanding that there is quite a bit of variation, especially within root structures) anatomical dentition in order to detect anomalies and be able to restore teeth to working form.