#25 What I Learned Today

Professionalism and Ethics

We broke into groups today and worked at putting together a code of ethics that we wanted our class to follow. It was pretty open-ended, ranging from things like specific do's-and-do-not's, to personal characteristics to follow.

When the code is put together and made available on-line, I'll be sure to post it here.

Biochemistry

We continued our look at energy metabolism by moving from the Kreb's cycle to oxidative phosphorylation and the electron transport chain. In summary, this step takes the hydrogen transporters, releases their hydrogens via oxidative phosphorylation. This sets up a hydrogen gradient, which the enzyme ATP synthase uses to produce ATP molecules.

The first step is to get NADH produced outside the mitochondria by glycolysis into the cell so that it can be processed by the enzymes in oxidative phosphorylation. Since the transporters cannot move freely across the membrane, two specific shuttles are put to good use:

Glycerol Phosphate Shuttle
Both the cytoplasm and mitochondria share an identical reaction: dihyroxyacetone phosphate can be converted to glycerol phosphate. In the cytoplasm, this oxidizes NADH to NAD+. Glycerol phosphate in then shuttled across the membrane and into the mitochondria, where it is converted back to dihydroxyacetone phosphate. This reaction reduces FAD+ to FADH2. However, as you probably know, FADH2 generates less ATP than NADH, so some energy is lost in this shuttle. The better way is to transfer the hydrogen from an NADH cytoplasm to a mitochondrial NADH. This is what the malate shuttle does.

Malate Shuttle
In the cytoplasm, oxaloacetate is reduced to malate, which converts an NADH to NAD+. Malate is then transported into the mitochondria, using a transporter which also sends alpha-ketoglutarate into the cytoplasm. Once in the mitochondria, malate is oxidized back to oxaloacetate, which converts NAD+ to NADH. Oxaloacetate then undergoes transamination with glutamate to become aspartate, where it is shuttled out to the cytoplasm. The aspartate then undergoes transamination with alpha-keoglutarate to regenerate the oxaloacetate and convert the A-K to glutamate.

Electron Transport Chain
Energy from the electron transporters then form the electrochemical hydrogen gradient by releasing H+ in a step-by-step oxidative phosphorlation process. The five components of the process include 4 oxidoreductases, and ATP synthesis.

Step 1: NADH transfers electrons to ubiquinone
Step 2: FADH2 transfers electrons to ubiquinone

The ubiquinone from these two separate steps come together to continue with grouped oxidative phosphorylation:

Step 3: ubiquinone transfers electrons to ferricytochrome b
Step 4: ferricytochrome transfers electrons to oxygen (to form water)
Step 5: ATP Synthesis

With ATP syntheis, hydrogen binds to the 'stalk' portion of ATP synthase, which is embedded in the membrame. This changes the shape and makes the 'wheel' portion go from wanting to bind ADP and inorganic phosphate to wanting to bind ATP. This shape changes produces sufficient energy to force ADP and Pi together to form ATP. As more hydrogen molecules pass through, the shape changes again and favours ADP and Pi binding again.