I am currently teaching nutrition and metabolism to first year medical students. The problem in this subject is the large number of individual reactions which leads students to think of the subject the way somebody described the study of history: just one damned thing after another. I try to present the big picture and the approach is the systems or “black box” strategy. The method is to ask whether we can get some information just by looking at the inputs and outputs to a system even if we don’t know any of the details of what’s going on inside. In other words, it is a way of organizing limited information. The method is favored by engineers who are the people most unhappy with the idea that they don’t know anything at all. First, the big principles.
Metabolism: two goals, two fuels.
There are two major goals in human energy metabolism: First, to provide energy for life processes in the form of the molecule ATP and second, to provide glucose for those cells that require glucose (particularly brain and central nervous system) and to maintain blood glucose at a relatively constant level: too little is obviously not good but too much is also a problem in that glucose is chemically reactive and can interact with body material, particularly proteins when at high concentrations. Of course, metabolism does many things but these are the two major goals in providing energy.
A second big generalization is that in this process there two kinds of fuels: glucose and acetyl-Coenzyme A (abbreviated acetyl-CoA or sometimes written as acetyl-SCoA; the S, which is meant to show that the compound contains sulfur, is not pronounced).
You knew what we do in metabolism even before you started reading this. Putting it into black box terms, you knew: we take in food and we take in oxygen. We excrete CO2 and water. Somehow this gives us the energy for life as well as the material to build up components of the body. You don’t have to know too much chemistry to figure out the important conclusion that, inside the black box, living systems use oxidation, just like combustion in a furnace. Lavoisier’s whole animal calorimeter that I described in a previous post was a beautiful real demonstration of this black box. More technically, this is an oxidation-reduction reaction. Oxidation, in a biochemical context, means combination with oxygen or loss of hydrogen and reduction means loss of oxygen or gain of hydrogen; we say that the (carbons in the) food gets oxidized and the oxygen gets reduced (to water). Like the common oxidation reactions you know (combustion in a furnace or an automobile engine), this produces energy which can be used to do work. Some work is mechanical work — moving muscles — but most of the energy is used for chemical: work making body material and keeping biological structures intact and generally keeping things running. The medium of energy in metabolism is the chemical reaction of synthesis and breakdown of the molecule ATP. Textbooks frequently refer to ATP as a “high energy molecule” but it is not the compound itself but rather the reaction (synthesis and breakdown (hydrolysis)) that is high energy. For the moment, we can think of ATP as the “coin of energy exchange in metabolism.” A heavy-duty thought concept: the challenge for biochemistry historically was to explain how the energy from an oxidation-reduction reaction could be used to carry out the synthesis of ATP which has a different mechanism (phosphate transfer). The process is called oxidative phosphorylation and was only figured out about fifty years ago.
So again, our two goals in human metabolism: Make energy in the form of ATP and maintain a pretty much constant level of blood glucose for those cells, brain and central nervous system, that require glucose (the brain can’t use fatty acids as a fuel).
Let’s look at energy production first because it is a little easier to understand. As we look inside the black box, each of the processes uncovered will have its own degree of complexity. In reading this you have to do what scientists do: hang in there. Skip over the parts that seem complex and see if you can come back to them later.
So, breaking into the black box, the first thing to notice is that the oxidation of food is done in steps, and that there is another player that mediates the process by coupling separate pieces: the food never sees the oxygen. The intermediaries are called coenzymes or cofactors. The most important oxidative coenzyme is known as NAD. It’s always referred to by the acronym, but if you’ve had some organic chemistry and you’re curious, NAD stands for nicotinamide-adenine-dinucleotide; the structure is shown in the figure and the action end of the molecule is indicated. NAD coenzymes are derived from the vitamin niacin. So what happens in metabolism is that food is oxidized by NAD+ (the oxidized form of NAD) and the product, NADH (the reduced form) is re-oxidized by molecular oxygen. Although it is still just as we thought (food+oxygen-in, CO2+water-out), the oxygen never sees the food. Why do we do it this way? If we did it all in one big blast like an automobile engine, we would have little control over it and we would not be able to capture the energy in a usable chemical form.
It’s easiest to start with glucose, a six-carbon compound. The early steps in metabolism involve a process known as glycolysis (sugar splitting) that ultimately gives you two molecules of a three-carbon compounds known as pyruvic acid. Pyruvic acid is oxidized to a derivative of acetic acid, known as acetyl-CoA. The CoA is short for Coenzyme A, a complicated molecule but, like many coenzymes is always referred to in this way so it is not important to know the detailed structure. The compound is frequently written acetyl-SCoA to emphasize that it is a thioester (sulfur ester); again, the “S” is not pronounced.
Acetyl-SCoA is the fuel for the major NADH-producing process, known as the Krebs cycle after the major player in its discovery. Without looking into that black box too much the key compound is citric acid, which is, chemically a try-carboxylic acid (TCA) so the Krebs cycle is also called the citric acid cycle or TCA cycle; Krebs called it the TCA cycle so I will generally use that term. The process whereby NADH is finally re-oxidized by oxygen is known as electron transport. So, The big black boxes of metabolism:
Where do we get Glucose and Acetyl-CoA?
So far we know: most energy comes from the oxidation of acetyl-CoA and most of the glucose that provides energy does so by first being converted to acetyl-CoA. Where else can we get acetyl-CoA? We’ve taken glucose as synonymous with food but where else can we get glucose from besides the diet?
indicates the extent to which the exceptions become important but the overriding principle that has the most impact on metabolism is that you cannot make glucose from fat).
Summary of fuel sources and synthesis and looking ahead.
- There are, roughly speaking, two kinds of fuels: glucose and acetyl-CoA.
- Carbohydrates and other nutrients, fat (that is, fatty acids) and protein (amino acids) can supply acetyl-CoA. Glucose is not required for acetyl-CoA and under conditions of low carbohydrate or low total food, fatty acids become the major source of acetyl-CoA.
- Not all tissues can use all fuel sources. Brain, CNS and red blood cells, for example cannot use fatty acids. Brain and CNS can use acetyl-CoA but cannot get it from fatty acids. Red blood cells only use glucose and, to a first approximation, brain and CNS are also dependent on glucose for metabolism.
- Under conditions of starvation or carbohydrate restriction, acetyl-CoA can be effectively transported from the liver in the form of ketone bodies. Ketone bodies, then, are a source of acetyl-CoA that can be used by brain and CNS. Red blood cells are still dependent on glucose but the brain’s demand for glucose is reduced by the availability of ketone bodies.
- There is no dietary requirement for carbohydrate and amino acids can also supply glucose through the process of gluconeogenesis.
- Fat as a source of acetyl-CoA also works the other way: acetyl-CoA can be converted to fat.
- Whereas glucose can be converted to fat, with a few exceptions, fat cannot be converted to glucose. This will be a key idea behind carbohydrate restriction.
- Glucose can also be stored as the polymer glycogen.
- Bottom line is the limitation of “you are what you eat.” Metabolism means the interconversion of food and metabolites. Conversely, it will be critical that not everything is interconvertible. In particular, we will emphasize that you can make fat from carbohydrate but, to a large extent, you cannot make glucose from fat.
Looking ahead on sources of blood glucose:
- Glucose from dietary input (referred to as the fed state; in nutrition, as the postprandial period), is depleted after about 8 hours.
- Glycogen is a storage/supply source of glucose. Liver glycogen can supply export glucose to the blood, thereby supplying other tissues. Muscle glycogen supplies glucose only for the muscle itself. Glycogen may become largely depleted after 24 hours, depending on the conditions (exercise, for example).
- The third source of blood glucose is gluconeogenesis (GNG) which, as the name implies, makes glucose anew from existing metabolites. Depending on the conditions, the source of carbon may be amino acids, lactic acid or glycerol from fat metabolism. Whereas it is sometimes indicated to be a “last ditch” source of glucose in the textbooks, it goes on all the time. The glucose it synthesizes in GNG may be used to replenish glycogen and only appear in the blood at a later time.