This lecture is meant to replace the planned lecture on regulating feeding behaviors. We will use this period to extend our treatment of nutrition by adding details on the biochemical roles of some vitamins. This will also serve as a review of energy metabolism, meant to help prepare for the final examination.

We recall that muscle contraction involves interactions between thick myosin filaments and thin actin filaments. These interactions occur by way of cross bridges formed by heads of the myosin molecules. Mg++ - dependent Myosin ATPase is responsible for the conformational changes in the orientation of the myosin cross bridges that result in the filaments sliding past on another. The sliding filaments result in a shortening of the Z-band to Z-band distance.

Tracheoles functionally, but not topologically, penetrate deep into rapidly metabolizing muscle fibers such that oxygen can be delivered nearly directly to muscle mitochondria, sometimes called sarcosomes. The key function of the gas exchange is to support energy metabolism during flight, or any other energy-intensive activity. By now you should have an idea of where, morphologically and biochemically, oxygen is consumed and where carbon dioxide is produced. We can revisit energy metabolism by looking back at the chart that shows types and forms of fuels.

We can also recall the metabolic pathways that yield energy to support cell work. These include glycolysis, Kreb's Cycle, the Electron Transport Chain and ß-oxidation. A physiologist might say, barring some needling details, your grasp on energy metabolism is virtually unrivalled in the western hemisphere.

The sugars, lipids and protein components that insects and other organisms consume in the course of their feeding activities are called macronutrients, not because they are particularly large or complex molecules, but because they are required in relatively large quantities. In addition to these macronutrients, insects require a host of micronutrients, so called because they are needed in more or less trace levels in the body. Some of these micronutrients are VITAMINS, which can be defined by three points. First, they are found in trace to low quantities in the body. Second, they are involved in specific metabolic functions. Third, they are not synthesized in the body. If they are synthesized, the synthetic rates do not support the organismal needs.

Some of the vitamins are directly involved in energy metabolism, acting within the pathways within our line of familiarity. We are going to look at these vitamins, and at the specific metabolic functions they perform. This is one of the areas of biochemistry that integrates common features of many different organisms. These metabolic functions are just as vital to mammals, including human beings.

Generally speaking, we recognize two major groups of vitamins - the fat soluble and the water soluble vitamins. The fat soluble group includes vitamins A, D, E, and K. The water soluble group includes the B-complex and vitamin C.

We proceed with vitamins we have already seen in action. The structure of riboflavin is shown first. This figure shows riboflavin in its vitamin form, as riboflavin, and also in its physiologically functional form in flavin adenine dinucleotide, often abbreviated as FAD. The key feature here is that the vitamin - that is, the trace substance that can not be manufactured within the body - is merely a component of a larger molecule that plays a specific role as a coenzyme. Before we discuss the role of FAD, let us introduce nicotinamide, also shown in its vitamin form and in its physiological form of nicotinamide adenine dinucleotide, similarly abbreviated as NAD.

Both FAD and NAD function as electron carriers. When an electron pair is transferred to these molecules, either one or two hydrogens are also transferred. FAD has accommodation for two hydrogens while NAD can accept one hydrogen. In NAD, then, an electron pair and one hydrogen are transferred, with a second hydrogen released into the medium. This subtle chemical difference is why the reduced form of FAD is written FADH2 while the reduced form of NAD is written NADH + H+. These compounds are particularly appropriate electron carriers because they readily accept an electron pair and they also readily donate an electron pair. We saw FAD act in the Kreb's cycle, shown in the figure. FAD accepts an electron pair from succinate, becoming FADH2. FADH2 carries the electron pair to the electron transport chain, where it releases its electrons at such position in the chain that two ATPs are subsequently formed. The figure also shows that NAD operates as an electron carrier at three places in the Kreb's cycle, carrying electrons from isocitrate, alpha-ketogluterate and from malate to the electron transport chain. Here the electrons are loaded onto the chain such that three ATPs are formed.

These examples illustrate a general feature of co-enzymes: they all provide some chemistry to enzymes that the proteins can not perform on their own. As coenzymes, FAD and NAD provide the ability to accept and donate electrons. Riboflavin and nicotinamide are necessary components of these co-enzymes, and these components can not be made in the body of insects. For perspective, recall from your biochemistry that NAD acts as a co-enzyme in more than 250 enzyme functions of this sort: carrying electrons from one place to another.

Our next water soluble vitamin is B-1 or thiamin, shown in its vitamin form and in its physiologically active form as thiamin pyrophosphate, or what is sometimes called cocarboxylase. This vitamin is quite often referred to as TPP. As a point of history, it was because of the amine group in this compound that accessory food factors were first called vitamines, an abbreviation of vital amines. Specifically, TPP is a coenzyme in the removal of carbon dioxide from various compounds. An important reaction is this one:

Pyruvate - -> Acetyl CoA + CO2

Recall that this reaction is carried out by pyruvate decarboxylase. This reaction removes the carbon dioxide moiety from pyruvate, and it connects the glycolytic pathway to the Kreb's cycle.

Returning to the Kreb's cycle, note that carbon dioxide is released at two points in the cycle. The active form of thiamin, TPP, also functions in the release of this second carbon dioxide. We collect the point that some decarboxylations, or carbon dioxide removal reactions, do not depend the activity of this co-enzyme.

We have now seen three vitamins - two function as electron carriers, essentially connecting the Kreb's cycle to the electron transport chain and the third functions in decarboxylation reactions. In one decarboxylation reaction, specifically pyruvate decarboxylase, TPP also helps connect the glycolytic pathway to the Kreb's cycle. The transport of 2-carbon acetyl units, done by Co-enzyme A or CoA, is still another key link between glycolysis and the Kreb's cycle. Recall that oxidation of fatty acids in the ß-oxidation pathway is also connected to the Kreb's cycle by transfer of acetyl CoA. This serves to underscore the importance of vitamin B-3, or pantothenic acid. This is shown in its vitamin form and as a component of Co-enzyme A. These four vitamins - nicotinamide, riboflavin, thiamin and pantothenic acid - are directly involved in energy metabolism; they act as electron carriers, in decarboxylations and as an essential component of CoA.

Insects require other water soluble vitamins in their diets, including biotin and folic acid. These do not function specifically in energy metabolism. Nonetheless, we want to briefly consider their roles in cellular metabolism. The chemical structure of biotin was worded out in 1942, and first synthesized in 1943. Biotin had been independently discovered by at least two groups, and it went under a variety of names, including vitamin H and protective factor X. Biotin functions as a carbon dioxide carrier in carbon dioxide fixation reactions. Here is an example:

Folic acid is actually a complex of several biologically active compounds. The formal term is pteroylglutamic acid, and it is composed of pteroic acid plus one to seven gluatmate residues. Folic acid is required for transferring single carbon units. It is essential in metabolism of some amino acids, and is probably most well known for its critical role in synthesis of thymine. Without folic acid, DNA synthesis is slowed or stopped. In the absence of DNA synthesis, production of some cells, such as red blood cells, is also slowed or stopped. This is particularly important for cells that undergo rapid turnover, such as red blood cells or insect hemocytes.

I think it is appropriate to provide a statement as to what level of detail is subject to examination. You should know the common names of the vitamins involved in energy metabolism and what their physiologically active forms are. You should be able to describe the general sort of chemistry they provide and be able to give a specific example. I would not want you to give much thought to the chemical structures of things or try to memorize many more than about half of the 250 know NAD reactions.

With respect to the water soluble vitamins, insects appear to be closely aligned with known vertebrate nutritional requirements: both groups require about the same vitamins for the same biochemical reactions. This is reasonable because the major energy yielding pathways are similar in both groups.

The fat soluble vitamins, vitamins A, D, E, and K, are not involved in the oxidative pathways we have discussed. Vitamins D and K are involved in functions occurring in vertebrates, but not invertebrates. Vitamin D is involved in bone metabolism, and vitamin K is involved in the vertebrate blood clotting cascade. Vitamin K was discovered in 1929 by a Danish scientist, Henrik Dam, who later won a Nobel prize for the work. The K comes from the Danish word koagulation. The long-standing wisdom holds that insects or other invertebrates do not require vitamins D and K.

Vitamin A is involved in many mammalian tissues, mostly in ways not completely understood. One well-appreciated action of this nutrient is its role in vision. We will discuss the physiology of vision in a later lecture, in which the role of vitamin A will be addressed in detail. For now we will just make the point that vitamin A is required for insect vision. Vitamin E is also thought to be dietarily essential in at least some insects. Vitamin E is essential for fertility in rats, and apparently in male crickets. The specific roles of vitamin E are not understood, although its most well known activity is its action as an antioxidant. Vitamin E is thought to interact with certain free radicals, and thereby prevent oxidative damage to cells. Whether vitamin E is required by all insects is difficult to say. I speculate it is required, however, the requirement is very hard to detect because the vitamin is stored in tissues and turns over only very slowly.

Now there is one last problem in energy metabolism. Referring to the glycolytic pathway again, please note that at one point NAD picks up a pair of electrons to become NADH + H+. There are two points to be made: One, if the electrons were not subsequently donated from the NADH, cells would eventually become depleted of NAD - and energy metabolism would come to a screeching halt right at this step. So it is important to recycle this electron carrier to keep energy metabolism rolling along. The second point is that this pair of electrons represents a certain amount of potential energy in the form of additional ATPs if the electron pair could be shuffled over to the electron transport chain. Stated in problematic terms, we have this problem: the outer mitochondrial membrane is not permeable to NAD. There are two pools of NAD in most cells. One is the inner mitochondrial pool and the other is the cytoplasmic pool. Since the electron transport chain is tucked away in the inner membrane of the mitochondria, how is the electron pair, associated with the cytoplasmic pool of NAD, to be delivered to the electron transport chain, or is it delivered at all?

Let us consider the operation of the glycerol-3-phosphate shuttle. On the left is the glycolytic pathway. This is in the cytosol fraction of the cell, where the NAD accepts an electron pair and becomes NADH (plus H⁺). The electron pair is transferred to dihydroxyacetone phosphate (DHAP). This step regenerates the NAD, thereby allowing the glycolytic pathway to continue its important operations. Reducing the DHAP by addition of electrons converts it from DHAP to glycerol-3-phosphate. This newly formed compound easily migrates across the outer mitochondrial membrane. Again, the membrane is a barrier to NAD flux. Once in the mitochondria, the electron pair is donated to the electron transport chain at the level of FAD, which converts the glycerol-3-phosphate back into DHAP. DHAP is free to wander back out into the cytosol. Then two points are covered by the glycerol-3-phosphate shuttle: the NAD is regenerated and the potential ATPs are realized.

This shuttle is best known from insect flight muscles. There are other pathways to account for moving reducing equivalents from cytosol into mitochondria, one of which is called the MALATE SHUTTLE because malate is formed by oxidation of oxaloacetic acid. The malate moves into the mitochondria, where it is converted to fumarte and then into succinate. This is a more complex shuttle, and I once speculated at a scientific meeting that the glycerol-3-phosphate shuttle is much faster than the malate shuttle, hence more appropriate to very rapidly metabolizing tissues, such as insect flight muscle.