Archive for the ‘energy metabolism’ Category

Discussions on energy balance and diet have not improved over the years. Most of social media and even the medical literature pretty much conform to what is called, in communications, half-duplex, and tends to generate, as they say, more heat than light. What remains interesting, however, are the scientific points associated with metabolic inefficiency,


Many biological reactions function in a steady state cycle of synthesis and breakdown. In adipocytes (fat cells), for example, there is a continuous cycle of synthesis of fat (triacylglycerol, TAG) and lipolysis (break-down) that goes on all the time. The overall reversible reaction:    3 fatty acid + glycerol ⇌  triacylglycerol + 3 H2O

Fatty acid from the hydrolysis of TAG (or fatty acid from the circulation) is processed for energy(ATP is generated).BLOG_TAG-FA_CYCLE_OxidationFA

The lipolysis (breakdown) step goes by itself but to re-synthesize TAG constitutes an uphill reaction (requires energy) — it’s easy to break stuff. If you want to make things, it costs you. So to put the fat molecule back together, you have to transform the fatty acid and glycerol molecules to make them more reactive. The actual substrates are glycerol-3-phosphate and fatty acyl-CoA which are more chemically reactive but you have to get the energy from someplace, so the synthesis of these compounds requires ATP. This is how fat becomes stored from fatty acid coming into the adipocyte. Glycerol-3-phosphate can be made in the liver from the glycerol from a previous round of lipolysis but, in adipose tissue, the glycerol-3-phosphate comes indirectly from a series of reactions. It is currently believed that the main one is glyceroneogenesis, the truncated form of gluconeogenesis, although some may come from glycolysis.

“glycerol” (different sources of glycerol molecule) +  ATP  → glycerol-3-P + ADP + H2O.

fatty acid + CoA-SH +  ATP → fatty acyl-CoA  + AMP + 2 phosphate +  H2O.BLOG_FA-TAG_TAG_SynthThere is thus a steady-state that continuously readjusts levels of fat and fatty acid. The process will drift in the direction of oxidation when stored fat provides energy to other cells and will tend in the opposite direction — toward synthesis — when fat is stored. The important point is that the steady-state, like an equilibrium state, does not mean that everything has stopped. It means that the forward rate of breakdown is equal to the resynthesis rate. Every time there is a cycle, TAG → FA → TAG,  however, energy is wasted — synthesis of TAG requires ATP, lipolysis is spontaneous and no ATP is re-syntesized. Why would such a thing evolve? The common name of the process is substrate cycle but because each cycle wastes ATP and accomplishes nothing — you get back the substrate that you started with — it has been referred to as a “futile cycle.” Why would the adipocyte waste energy in this way?

The energy in the TAG-fatty acid cycle is not really wasted even though it looks that way on paper. It improves overall efficiency. The cycle regulates the availability of energy to the body. As such it must be able to respond to differing conditions rapidly. Regulation is easier if competing reactions are maintained in a cycling steady-state and then biased in one or another direction. This becomes, in the end, more efficient  than starts and stops in response to different conditions. The TAG-FA cycle :

BLOG_TAG-FA_LOOPDiesel engines

I usually describe, as an analogy, how, if you walk past a bus station, you might see that the buses are parked with their engines idling. Probably less common now than it used to be, the explanation was that it is difficult to start a diesel engine and it is more efficient to let it idle and then put it in gear. Fuel costs and engine designs have changed since the analogy first occurred to me so I checked on line. There is now some controversy and some of the discussion is reminiscent of Marissa Tomei’s testimony in My Cousin Vinny but it is still true that it is common to let diesel engines idle when parked for reasonable periods of time. Diesel engines don’t have spark plugs and depend on high compression and generate high temperatures. It is costly to start and stop the engine repeatedly.  The analogy is that is more efficient to run a cycle of metabolic reactions and then readjust which direction you want it to go in than to start and stop.


The point is that you will store different amounts of fat depending on how many cycles you run in a given amount of time. For weight loss, of course, you hope to run as inefficiently as possible (relative to fat storage. The “wasted energy,” however, is less than if you had a lot of starts and stops.

To determine lipolysis in the adipose tissue, you can measure the appearance of fatty acid in the blood. If the process is simple, that is, if only lipolysis is going on, then the stoichiometry (balance of reactants and products) should be 3:1, three fatty acid molecules for every glycerol released.  If, however, the fatty acid is re-processed, more or less fatty acid will appear in the blood compared to the amount of glycerol that is produced. You can then calculate the rate of cycling = 3x (rate of glycerol appearance) – (rate of FA appearance).

The rate of cycling is increased by feeding, turned on by adrenergic stimulation (norepinephrine), turned on by glucagon and turned off by insulin.BLOG_FA-TAG_CYCLE_MAR_29

Whether, and to what extent this figures into metabolic efficiency and CICO seems like a good question. Anyway, here’s picture of the main inputs and outputs:


This series of posts is a followup to the project that Dr. Eugene Fine and I described in our campaign at as follow-up to Dr. Fine’s pilot study of ten advanced cancer patients on ketogenic diets and the in vitro projects that we are carrying out in parallel.

The last post described the two major processes in energy metabolism, (anaerobic) glycolysis and respiration. Pyruvate is the product of glycolysis and has many fates. (Remember pyruvate and pyruvic acid refer to the same chemical). For cells that rely largely on glycolysis, pyruvate is converted to several final products like ethanol, lactic acid and a bunch of other stuff that microorganisms make in the fermentation of glucose. (The unique smell of butter, e.g., is due to acetoin and other condensation products of pyruvate). Rapidly exercising muscles also produce lactic acid.

The sudden interest in the metabolic approach to cancer stems from the work of Otto Warburg whose lab in the 1930’s was a center for the study of metabolism. (Hans Krebs was an Assistant Professor in the lab). Warburg’s landmark observation was that cells from cancer tissue showed a higher ratio of lactate to CO2 than normal cells, that is, the cancerous tissue was metabolizing glucose via glycolysis to a greater degree than normal even though oxygen was present. The Coris (Carl and Gerty of the Cori cycle) demonstrated what is now called the Warburg effect in a whole animal. Ultimately, Warburg refined the result by comparing the ratio of lactate:CO2 in a cannulated artery to that in the vein for a normal forearm muscle. He compared that to the ratio in the forearm of the same patient  that contained a tumor. Warburg claimed that this greater dependence on glycolysis was a general feature of all cancers and for a long time it was assumed that there was a defect in the mitochondrion in cancer cells. These are both exaggerations but aerobic glycolysis appears as a feature of many cancers and defects in mitochondria, where they exist, are more subtle than gross structural damage. The figure shows current understanding of the Warburg Effect.


What about this mechanism makes us think that  ketone bodies are going to be effective against cancer? We need one more step in biochemical background to explain what we think is going on. In normal aerobic cells, pyruvate is converted to the compound acetyl-CoA.  Acetyl-CoA represents another big player in metabolism and functions as the real substrate for aerobic metabolism. If you have taken general chemistry, you will recognize acetyl-CoA as a a derivative of acetic acid.

The reaction acetyl-CoA ➛ 2CO2 is the main transformation from which we get energy. Acetyl-CoA provides the building block for fatty acids and for ketone bodies. Conversely, fatty acids are a fuel for cells because they can be broken down to acetyl-CoA. Under conditions of starvation, or a low-carbohydrate diet, the liver assembles 2 acetyl-CoA’s to ketone bodies (β-hydroxy butyrate and acetoacetyl-CoA). The ketone bodies are transported to other cells where they are disassembled back to acetyl-CoA and are processed in the cell for energy. The liver is a kind of metabolic command center and ketone bodies are a way for the liver to deliver acetyl-CoA to other cells.kdforca_blog-iii_dec_4

Now we are at the point of asking how a cell knows what to do when presented with a choice of fuels? In particular, how does the input from fat dial down glycolysis so that pyruvate, which could be used for something else (in starvation or low-carb, it will be substrate for gluconeogenesis), is not used to make acetyl-CoA.  It turns out that acetylCoA (that is, fat or ketone bodies) regulate their own use by feeding back and directly or indirectly turning off glycolysis (in other words: don’t process pyruvate to acetyl-CoA because we already have a lot). The feedback system is known as the Randle cycle and appears (roughly) as the dotted line in our expanded metabolic scheme.

robin_map_2012-2Where we are going. In our earlier work Dr. Fine and I and our assistant, Anna Miller, found that if we grow cancer cells in culture, acetoacetate (one of the ketone bodies) will inhibit their growth and will reduce the amount of ATP that they can generate. Normal cells, however, are not inhibited by ketone bodies and the cells may even be using them. Our working explanation is that the ketone bodies are inhibiting the cancer cell through the Randle cycle. Now, normal cells can maintain energy, that is compensate for the Randle cycle, by running the TCA cycle (in fact, that is the purpose of the Randle cycle: to switch fuel sources). The cancer cells, however, have some kind of  defect in aerobic metabolism and can’t compensate.  How does this happen? That’s what we’re trying to find out but we have a good guess. (A good guess in science means that when we find out it’s wrong we’ll probably see a better idea). We find that our cancer cells in culture over-express a protein called uncoupling protein-2 (UCP2). We think that’s a player. To be discussed in Part IV.



























Metabolism: two goals, two fuels.  



The black box of life. 




The role of redox coenzymes




Where do we get Glucose and Acetyl-CoA?



Summary of fuel sources and synthesis and looking ahead.

  8. Glucose can also be stored as the polymer glycogen.

Looking ahead on sources of blood glucose:
