#17 What I Learned Today

Biochemistry

We started reviewing Kreb's Cycle (aka TCA cycle, citric acid cycle) in depth. To summarize, it takes acetyl CoA, adds an acetyl group to oxalocetate, and cycles this compound through several forms. In doing so, it releases 2 carbons worth of carbon dioxide (in order to maximize energy output), removes hydrogens to form 3 NADH and 1 FADH2 (which is used later in the electron transport chain to create water), and releases 1 GTP molecule. By the end of the citric acid cycle, all the carbons from the initial glucose have been converted to carbon dioxide.

Before Kreb's cycle occurs, the pyruvate molecule needs to be converted into acetyl CoA. This is performed mainly through the enzyme pyruvate dehydrogenase, and a bunch of different cofactors (including thiamin dehydrogenase, lipoic acid, and Coenzyme A). I won't bore you with specifics about the reaction, but ultimately, you get:

pyruvate + NAD + Coenzyme A -> CO2 + acetyl CoA + NADH

The citric acid cycle begins with the formation of citrate. This is catalyzed by the enzyme citrate synthase. An acetyl group is removed from coenzyme A and added to oxaloacetate. This step is physiologically irreversible.

The citrate is then converted to isocitrate via a two-step mechanism that is catalyzed by acontiase. A double-bond is added to citrate to form cis-aconitate, and then the end-carbon is oxidized to form the alcohol isocitrate.

Isocitrate is then converted to alpha-ketoglutarate, which is also done in two steps. Hydrogens are removed by NAD+ to form NADH (for usage in ETC later on), forming an unstable intermediate. A carboxylic acid is then removed in the form of carbon dioxide, which results in alpha-KG. This process is also physiologically irreversible.

You get the formation of more NADH and carbon dioxide when alpha-ketoglutarate is oxidized (more NAD+ to NADH). Another carboxylic acid is removed, to form more carbon dioxide. The end product is succinyl-CoA. At this point, all carbons have been fully oxidized to carbon dioxide at this point.

Finally, more hydrogens are removed from succinyl-CoA, to form GTP (an analog to ATP). This returns the carbon molecule back into oxaloacetate; its original form.

Principles of Dental Materials

Midterm today. Nothing was lectured.

Dental Anatomy

We went over mandibular premolars. I will cover this more tomorrow.