Started off with a revision of the structure of the F1ATPase – going over the c-subunit rotations, the gamma subunit rotations and the interaction with the alpha/beta subunits.

The main points were:

a)it takes 10 protons to turn the gamma subunit by a full rotation

b)the gamma subunit sits in a pocket made by the 10 c-subunits – rather like a hip joint. Indeed all the residues in this pocket are really hydrophobic so it’s even ‘greased’!!!

c)the gamma subunit is asymmetric and changes the conformation of the alpha- and beta-subunits as it interacts with them - imagine it to be a bit like the pokey out bit on the camshaft of your car engine

d)the alpha-beta portion is kept from rotating by a group of proteins that make up the stator. Remember that without this stator, turning the gamma subunit would simply turn the whole alpha-beta portion…. The aim is, of course, to have RELATIVE movement between the two portions.

Then we discussed how the ATP is formed by the beta-subunits

The three beta subunits change in conformation as the gamma-subunit whizzes past them… first binding substrate, then closing in around the substrates to make ATP, then releasing the ATP. Each of the three beta-subunits do each step once during a full revolution of the gamma subunit… so 3ATPs are made per revolution.

It takes 10 protons to do one revolution… and one NADH to pump out those 10 protons… so in theory, one gets 3ATPs per NADH.

P574-574 in Voet and Voet are strong on all this.

You’d get less for an FADH2 – wouldn’t you 

However, since protons are used in transporting ADP, phosphate and pyruvate into the mitochondria, and since there is always some ‘leakage’ through UCPs the EXACT stoichiometry is more like 2.5 ATP per NADH.

We then discussed the relative returns of oxidative phosphorylation and cytosolic substrate level phosphorylation.

a)Substrate Level Phosphorylation by glycolysis is relatively inefficient – only 2 ATPs per glucose – but it can occur very fast

b)Oxidative phosphorylation is much more efficient (>30 ATPs per glucose) but requires mitochondria and a good oxygen supply.

c)In tissues with few mitochondria (eg, red blood cells) or in tissues exposed to anoxia (lack of oxygen), the ATP must be generated by glycolysis. This means that the rate of glucose utilization must increase some 15-fold

d)This increase in glucose use when using glycolysis alone is called the Pasteur Effect (Section 14-3C in Voet and Voet)

e)Section 17.4D and the Box 17-5 in Voet and Voet are very good on this.

We then discussed the principle of ENERGY CHARGE. The idea that when ATP is low, two ADPs can be ‘combined’ to give an ATP and an AMP. The equilibrium constant of this reaction (catalysed by adenylate kinase) ‘amplifies’ the AMP rise – so that a small change in ATP and ADP is reflected by a very large change in AMP. This means that enzymes that respond to AMP are particularly good at responding to subtle changes in ATP.

p455 (bottom of the page) in Voet and Voet is talks about how adenylate kinase manages to translate a small change in ATP to a large change in AMP.

NEW TOPIC… THE DISPOSAL OF CARBOHYDRATE

Notice how I missed out all the stuff from last year on carbohydrate digestion and absorption. We may get this later.

So we start with the concept of EUGLYCEMIA – this is already well detailed in the PowerPoint from L7 last year. Section 21.1A (p746) of V&V has something on brain glucose demands

The most important concept is that the glucose that gets taken up into cells will either be stored as glycogen (glycogenesis) or will be oxidized (glycolysis, TCA cycle, oxidative phosphorylation).

Knowing which fate will be chosen is a matter of determining the activity of the SLOWEST enzyme in each of the two pathways. A pathway can only go as fast as its slowest enzyme. This is called the FLUX GENERATING or RATE LIMITING step. (See Section 13-1D -p402-404 of Voet and Voet)

Further properties of these controlling steps are that they tend to be saturated with substrate (ie, they are working at massive substrate concentrations… certainly > Km), they are irreversible and slow. Conversely, most of the enzymes in pathways tend to be ‘equilibrium’ enzymes that work at about their Km and are very fast in both directions.