|Lecture Notes: 2 April||
Last time we looked over the Kreb's Cycle, and finished with citrate synthesis:
Unfortunately the resulting citrate is a tertiary alcohol which cannot be readily oxidized. Aconitase catalyzes the rearrangement of citrate to give an oxidizable secondary alcohol. This reaction involves an elimination/addition sequence, catalyzed by an iron-sulfur cluster (Fe4S4), with an alkene intermediate, cis- Aconitate:
We have now converted the 3° alcohol, citrate, into an oxidizable 2° alcohol, isocitrate. The next reaction is the first oxidation of the TCA Cycle.
The isocitrate alcohol can now be oxidized with NAD+ by Isocitrate DH to give an enzyme bound intermediate. The intermediate has a carboxyl group beta to a carbonyl carbon, so it has an excellent leaving group, CO2, attached to a stabilized carbanion. Thus it immediately rearranges to lose carbon dioxide:
The resulting 2-oxo-glutarate (-ketoglutarate) looks just like pyruvate with an R-group attached to the -carbon, so it is broken down by a DH Complex, the -Ketoglutarate DH Complex, just as pyruvate was. This gives succinyl-CoA and releases a second carbon dioxide. Note that at this point two carbons have been released, so formally, we have released the two carbons of Acetyl-CoA (Though neither of them came from the acetyl CoA we added)! We have also produced two NADH's (4 NADH/Glucose) which will result in the production of 5 ATP's (or: 4 x 2.5 ATP/NADH= 10 ATP's/Glucose). However, we have not regenerated the carrier. The remainder of the cycle is involved in this regeneration. (Note in the first four reactions two carbons have been lost as CO2 - as many carbons have been lost as were picked up with acetate. In a sense the rest of the cycle is regenerating our carrier - oxalacetate!)
Succinyl-CoA, like acetyl-CoA, has a high-energy bond. However in this case the energy will be captured, using Succinyl-CoA Synthetase, to give a GTP which is energetically equivalent to an ATP (2 ATP's/Glucose). The mechanism of this reaction first involves the phosphorolysis by inorganic phosphate of the thiol ester bond to give a phosphoric-carboxylic mixed acid anhydride, followed by formation of a phosphorylated enzyme and finally transfer of the phosphate onto GDP.
The reactions beginning with succinate are representative of a common pattern, the "Mainline Sequence," seen repeatedly in biochemical pathways.
First, Succinate DH, an inner-mitochondrial membrane-bound enzyme and member of the mitochondrial electron transport system (ETS), oxidizes succinate to fumerate. This reaction uses the stronger oxidizer FAD as an oxidizing agent because of the added difficulty in oxidizing an alkane to an alkene. As a consequence of using this more powerful oxidizing agent, less ATP energy can be captured in oxidizing the resulting FADH2 with oxygen (FAD is closer to oxygen in its oxidation potential). One and one-half ATP equivalents are obtained in this reaction (or: 2 x 1.5 ATP/FADH2= 3 ATP/Glucose).
The resulting alkene, Fumerate, is not readily oxidized. However, if water is added across the double bond an alcohol results which can be oxidized. Thus Fumerase catalyses a hydration reaction to give malate.
Finally, Malate DH catalyzes the dehydrogenation of malate to regenerate the original carrier, oxaloacetate, and finish the cycle. In addition another NADH is formed (and 2 x 2.5 ATP/NADH= 5 ATP/Glucose).
For the entire cycle we then have the production of 10 ATP/acetyl-CoA or 20 ATP/Glucose. The aerobic catabolism of glucose can then give a maximum total of 32 ATP/glucose as summarized in the Table:
Note that because all catalysts (oxaloacetate, enzymes etc.) must be regenerated in looking at the overall operation of the cycle, only the acetyl group of acetyl-CoA can be oxidized completely. Some intermediates, such as citrate, can be partially oxidized, but Kreb's cycle intermediate catabolism requires leaving the cycle at oxaloacetate and then returning as acetyl-CoA. Note that this requires leaving the mitochondria for some reactions, and since the extremely low concentrations of oxaloacetate don't allow its efficient transport across the mitochondrial membrane (the Km of the carrier is much higher than [oxaloacetate]), malate is the species which actually leaves the mitochondria.
In order to understand the regulation of the TCA cycle we need to look at the G values for the various reactions and the kinetic properties of the enzymes. Values for the non-equilibrium reactions are tabulated below:
Note that normally the concentrations of ATP, ADP, NAD+, and NADH are relatively constant in the mitosol and are thus unlikely to be very effective as allosteric regulators under most circumstances. On the other hand the availability of NAD+ and FAD as substrate will affect the rate not only of the reactions in the table, but also the near-equilibrium dehydrogenases. Note that NAD+ availability in turn is determined by the activity of the electron transport system, whose activity is closely coupled to the availability of ADP. Thus high [ATP] will slow the TCA cycle since high [ATP] means low [ADP], which will slow the ETS resulting in low [NAD+]!
In muscle, Ca2+ does show significant changes in concentration in the mitosol (recall that an increase in [Ca2+] concentration initiates muscle concentraction). Succinyl CoA will also show significant concentration changes under differing conditions and can thus also serve as an effective regulator, indicating carbon status in the second half of the cycle.
Regulation and Control: The concentration of citrate also affects PFK activity as a negative effector.
Interconversion of metabolic intermediates: The TCA cycle has a central place in metabolism (even in anaerobic organisms) via its use to interconvert metabolites as summarized in Figure 13.13 in your text. We will look further at these interactions as the course proceeds.
Last modified 2 April 2007