Metabolic States of the Body

The body exists in a dynamic catabolic-anabolic state as organic molecules are continuously broken down and rebuilt – frequently at a head-spinning rate.
The blood transports different kinds of energy sources – glucose, ketone bodies, fatty acids, glycerol, and lactic acid. Some organs routinely use energy from sources other than glucose, which saves glucose for tissues that heavily rely upon it. Metabolic controls act to equalize blood concentrations of energy sources between two nutritional states of our body: the absorptive state, or the fed sate; and the post-absorptive sate, or the fasting state. The absorptive state is the period during and shortly after eating, when nutrients are flushing into the blood from the gastrointestinal tract. The post-absorptive is the period when the alimentary canal is empty and the energy sources are supplied by the breakdown of body reserves.

Absorptive State – Overview
During the absorptive state, anabolism exceeds catabolism. Dietary amino acids and fats are used to remake degraded body protein or fat, and small amounts are oxidized to provide ATP. The excess metabolites, regardless of the source, are transformed to fat if not used for anabolism.
Absorptive State – Carbohydrates
Absorbed monosaccharides are delivered directly to the liver, where fructose and galactose are converted to glucose. Glucose is then released to the blood or is converted to glycogen and fat. Glycogen formed in the liver is stored there, but most fat synthesized in the liver is packaged with proteins as very low density lipoproteins (VLDLs) and released to the blood to be picked up for storage by adipose tissue. Bloodborne glucose not sequestered by the liver enters body cells to be metabolized for energy, and any excess is stored in skeletal muscle cells as glycogen or in adipose cells as fat.
Absorptive State – Triglycerides
Nearly all products of fat digestion enter the lymph in the form of chylomicrons, which are hydrolyzed to fatty acids and glycerol before they can pass through the capillary walls. Lipoprotein lipase, is an enzyme that catalyzes fat hydrolysis, and it is particularly active in the capillaries of muscle and fat tissues. Adipose cells, skeletal and cardiac muscle cells, and liver cells use triglycerides as the primary energy source. When dietary carbohydrates are limited, other cells begin to oxidise more fat for energy. Although some fatty acids and glycerol are used for anabolic purposes by tissue cells, most enter adipose tissue to be reconverted to triglycerides and are stored.
Absorptive State – Amino Acids
Once absorbed, the amino acids are delivered to the liver to be deaminated and transformed to keto acids. The keto acids can flow into the Krebs cycle to be used for ATP synthesis, or they may be converted to liver fat stores. Some amino acids are used to synthesize plasma proteins, clotting proteins, and transport proteins. However, the majority of the amino acids flushing through the liver remain in the blood for uptake by other body cells, where they are used for protein synthesis.
Absorptive State – Hormonal Control
Insulin essentially directs all of the events of the absorptive state. Insulin is a hypoglycemic hormone. It sweeps glucose out of the blood and into the tissue cells, lowering blood glucose levels. Additionally, it enhances glucose oxidation or storage while simultaneous inhibiting any process that might increase blood glucose levels.
Rising blood glucose levels (after a carbohydrate meal) act as a humoral stimulus that prods the β cells of the pancreatic islets to secrete more insulin. (The glucose induced stimulation of insulin release in enhanced by the gastrointestinal tract hormone glucose-dependent insulinotropic peptide (GIP) and parasympathetic stimulation.) A second important stimulus for insulin release is elevated amino acid levels in the blood.
Insulin binding to the target cells’ membrane receptors stimulates the translocation of an important glucose transporter (GLUT-4 in muscle and adipose tissue) to the plasma membrane. GLUT-4 enhances the carrier mediated facilitated diffusion of glucose into those target cells. (Without insulin stimulation glucose cannot enter the target cell.) However, brain and liver cells taken in glucose whether insulin is present or not. Insulin also enhances the stimulation of active transport of amino acids into the cells, promotes protein synthesis (as long as the essential amino acids are present), and inhibits liver export of glucose and inhibits virtually all liver enzymes that promotes gluconeogenesis.
Once inside the cell, insulin enhances glucose oxidation for energy and stimulates its conversion to glycogen, and in adipose cells, to triglycerides.

So, to summarise insulin: it allows for glucose entry into the cells; enhances glycogenesis, inhibits glycogenolysis; stimulates lipogenesis, inhibits lipolysis;

Generic Protein Metabolism

Introduction
Amino acids are very important because they form all of the protein structures and form the functional molecules. Protein synthesis occurs on ribosomes, where the amino acids are combined by the ribosomal enzymes via peptide bonds into various protein polymers. The amount and type of protein synthesis is controlled by hormones (growth hormones, thyroxine, sex hormones, insulin-like growth factors, etc.). Therefore, a “healthy” protein synthesis reflects a hormonal balance.
It is not necessary to over-consume proteins. The needed, nonessential, amino acids can be synthesized by siphoning a keto acid out of the Krebs cycle and processing it. The required amine group will be added to the siphoned keto acid to transform it into the nonessential amino acid. This reaction is fully reversible and it takes place in the liver.
However, the essential amino acids are required to be supplied by the diet for protein synthesis to occur. In the absense of the essential amino acids, the body will oxidize all of the amino acids even though the ingested amino acids may be needed for anablism. Thus, it is an all-or-none phenomenon. In an absence of essential amino acids, the body will catabolize the ingested body protein for energy instead of working on protein synthesis.

Oxidation of Amino Acids
Before amino acids can be oxidized for energy, they need to be deaminated. The amine group (NH3) must be removed in the process termed transamination. The original amino acid becomes a keto acid. A keto acid is simply an amino acid that has an oxygen in the place of the amine group. On a side note, if there is a sulfur present in the amino acid, it needs to be released prior to deamination.

Transamination
The original amino acid is reacted with, typically, alpha-ketoglutaric acid. During this reaction, the amine group is transferred to the alpha-ketoglutaric acid, creating glutamic acid. Meanwhile, the original amino acid becomes a keto acid. The reaction is fully reversible at this point.
Oxidative Deamination
Next, in the liver, the amine group is removed from the glutamic acid as ammonia and, thus, the alpha-ketoglutaric acid is regenerated. The ammonia molecule is combined with carbon dioxide to form urea and water. The urea is, then, released into the blood stream and released from the body as urine. The ease of liberation of ammonia from the glutamic acid is very important in the urea cycle. It promotes body’s ability to rid of blood borne NH3 that was produced by the intestinal bacteria.
Keto Acid Modification
Finally, the newly transformed amino acid is further modified to produce the necessary metabolite that can enter the Krebs cycle. The most important metabolites are: pyruvic acid, acetyl CoA, alpha-ketoglutaric acid, and oxaloacetic acid. Deaminated amino acid that are transformed into pyruvic acid can be, also, modified to create glucose if needed via the gluconeogenesis process.

Generic Lipid Metabolism

Lipid Metabolism

The human body carefully regulates how much energy it wants to generate. Therefore, unfortunately or fortunately, unlimited amount of glucose do not equate in unlimited ATP synthesis. Rising intracellular ATP concentrations eventually inhibit glucose catabolism and initiate glucose storage as glycogen or fat. Due to preference, the cells are able to store more fat than glycogen. Thus, fat accounts for 80-85% of stored energy.

Fats contain very little water, and yield more energy from catabolism than proteins or glucose. Products of fat digestion are transported in lymph in the form of protein enclosed free fatty acid (FFA) droplets called chylomicrons.

Triglyceride Oxidation: Glycerol and Free Fatty Acid Chains

Of the various lipids, only triglycerides (TG) are routinely oxidized for energy. The catabolism of triglycerides results in two different building blocks: glycerol and free fatty acid chains. Glycerol gets converted to glyceraldehyde phosphate and enters the glycolytic pathway. (However, glyceraldehyde phosphate produces about half the energy that a simple glucose can produce.)

Meanwhile, triglycerides undergo beta-oxidation in the mitochondrion. Beta-oxidation refers to the oxidation of the carbon in the beta (third carbon) position during the process and cleavage of the fatty acid in each case occurs between alpha and beta carbons. The fatty acid chains are broken apart into 2-carbon acetic acid fragments (while, the FAD and NAD+ coenzymes get reduced). Coenzyme A gets fused with the acetic acid to form acetyl CoA which is a known starting material that the oxaloacetic acid picks up to begin the Krebs cycle. Acetyl CoA resulting from beta-oxidation of fatty acids cannot be used for gluconeogenesis because the process is irreversible (unlike glycerol subunit that directly enters the glycolysis).

Lipogenesis

Lipogenesis is the process of triglyceride synthesis. It occurs when the cellular ATP and glucose levels are high. (Excess ATP also leads to an accumulation of acetyl CoA that would otherwise enter citric acid cycle.) Glycerol and free fatty acid chains are recombined and stored for later usage. The accumulation of fatty tissue changes on the regular; and it is not of the same composition as it was a month ago.

Acetyl CoA molecules are condensed together. They form fatty acid chains, 2 carbons at a time. (Hence, why most of the fatty acids contain an even number of carbon atoms.) If you remember, acetyl CoA is an intermediate in glucose oxidation after the glycolytic pathway. Therefore, since the acetyl CoA is the starting material for fatty acid synthesis, glucose can be easily converted to fat. A diet poor in fat can utilise carbohydrates to provide all of the raw materials needed to form triglycerides. (High blood sugar results in lipogenesis as the major activity in liver and in adipose tissues.)

Lipolysis

The liver, cardiac muscle, and resting skeletal muscles actually prefer fatty acids as an energy source. Lipolysis is the breakdown of triglycerides into glycerol and free fatty acid chains. When carbohydrate intake is inadequate, lipolysis is accelerated as the body adjusts to fill the energy requirements from lipids. However, there is a limiting factor: oxaloacetic acid. The ability of oxaloacetic acid to act as a pick-up molecule is crucial for the acetyl CoA to enter the Krebs cycle. When there is a lack of glucose, oxaloacetic acid gets converted into glucose to be used as energy for brain functions. Without the oxaloacetic acid to act as a pickup molecule, acetyl CoA accumulates in the mitochondrion. However, the body has an adaptive response: ketogenesis. During ketogenesis, the liver converts the excess of acetyl CoA into ketone bodies, or ketones, which are then released to blood. (Ketone bodies, such as: acetoacetic acid, β-hydroxybutyric acid, and acetone.)

Acetyl CoA can be further processed into cholesterol in the liver; and the liver can use the cholesterol to form bile salts. The ovaries, testes, and adrenal cortex can use cholesterol to synthesise their respective steroid hormones.

So, what can be the problem with utilising ketone bodies? Ketone bodies are mostly organic acids, thereby lead to a decrease of blood pH. An accumulation of ketones in the blood is termed ketosis. Ketosis is a consequence of starvation, unwise dieting, and diabetes mellitus. It can lead to severe consequences and thus need to be looked into else where.

Macronutrient breakdown: Summary of ATP production

Summary

The cells need to produce around 100 kcal/hour of energy for an average human being at rest. For each 1 mole of glucose, about 686 kcal of energy is present. Our cells are able to capture 262 kcal in the ATP bonds. The remainder is liberated as heat. So, this corresponds to about 38% yield and 62% loss.

(Another way to view the above statement: 7.3 kcal of energy is produced by 1 mole of ATP molecule [7.3 kcal/mol]; 262 kcal / 686 kcal * 100% = 38% yield)

Each reduced NAD+ coenzyme gives off a pair of high energy electrons that contributes to a generation of about 2.5 ATP molecules. While, the oxidation of each FADH2 coenzyme is less efficient and leads to about 1.5 ATP molecules.

So, during glycolysis the cells generate 2 NADH + H coenzymes that yields 5 ATP molecules from the electron transport chain. 8 NADH +H and 2 FADH2 produced during Krebs cycle yields 20 and 3 ATPs via the electron transport chain, respectively. (Plus, 2 net ATPs produced during glycolysis and 2 ATPs in the Krebs cycle.)

However, there is an uncertainty about the energy produced from the reduced NAD+ that was produced outside of the mitochondrion. The reduced NAD+ cannot simply diffuse into the mitochondrial matrix across the inner mitochondrial membrane, but it has to utilise a shuttle molecule. Some cells appear to utilise different shuttle molecules that harvest the energy from the reduced NAD+ to fuel its transport. Thus, it appears that the net energy of reoxidation of the reduced NAD+ in such a case is equivalent to 1.5 ATPs per electron pair, just like for FADH2 oxidation. So, the net yield of ATPs per 1 glucose molecule will be 30 at the maximum.

There is another uncertainty about the 30 ATP production number due to the various processes that the cell does which are propelled by the electrons. (Such as bringing ADP, pyruvic acid, inorganic phosphate, etc. into the mitochondrial matrix.)

Macronutrient breakdown: Electron Transport Chain

Introduction

The hydrogen atoms that were removed during the oxidation of food are combined with O2 to form H2O. The energy that is released via the production of H2O is harnessed by the cell to attach an inorganic phosphate (Pi) to an ADP molecule to form ATP. The majority of the components of the electron transport chain (ETC) are proteins that are bound to metal atoms (also termed cofactors). These proteins vary in composition; and they combine to form multi-protein complexes that are embedded in the mitochondrial membrane. These complexes are alternately reduced and oxidized as they pickup electrons and pass the electrons to the next complex in the sequence. These complexes are called respiratory enzyme complexes. The complexes pump H+ into the mitochondrial intermembrance space that creates a gradient. This gradient provides energy to pump metabolites (ADP, pyruvic acid, inorganic phosphate) and calcium ions across the relatively impermeable inner mitochondrial membrane.

Electron Transport Chain

The hydrogen atoms are delivered by the reduced coenzymes NAD+ and FAD. The hydrogen atoms get split into protons (H+) and electrons. The electrons get shuttled along the inner mitochondrial membrane from one respiratory enzyme complex to the next. During this transfer, the electrons loose energy with each transfer; while, the protons escape into the watery mitochondrial matrix. However, the protons are immediately picked up by one of the respiratory enzyme complexes (I,III, or IV) and pumped across the inner mitrochondrial membrane into the mitochondrial intermembrane space.

The electron pairs are delivered to an oxygen atom, creating oxygen ions (O-). These ions very strongly attract H+ and form water.

The electrons must travel through the respiratory enzyme complexes in order to lower its energy levels (~53 kcal/mol). Otherwise, if a hydrogen, released by a reduced coenzyme, was to directly combine with an oxygen atom, then the energy would be released in one big burst. Thus, the energy would be mostly lost to the environment as heat instead of being captured and used by the cell.

The energy released during the travel of the electrons along the respiratory enzyme complexes creates is used to pump protons from the mitochondrial matrix into the mitochondrial intermembrane space. This is a significant process because the inner mitochondrial membrane is nearly impermeable to H+. Thus, this process creates a chemical gradient (or, more specifically, an electrochemical proton gradient) that has the potential energy and capacity to do other work. The presence of the electrochemical proton gradient does the following: (a) creates a pH gradient (due to the higher concentration of H+ in the mitochondrial intermembrane space than in the mitochondrial matrix); and (b) generates a voltage across the inner mitochondrial membrane (negative on the mitochondrial matrix side and positive between the inner and outer mitochondrial membranes). These two conditions create a strong “pull” or “attraction” of the H+ protons back into the mitochondrial matrix. However, due to the high impermeability of the inner mitochondrial matrix to protons, how can H+ enter the mitochondrial matrix?

The ATP Synthase is a large enzyme-protein complex that is freely permeable to the H+ ions. ATP synthase is located on the inner mitochondrial membrane. They have the structure of the nature’s smallest rotary motor. As the H+ ions travel along the ATP synthase they create an electrical current. ATP synthase is able to harness this electrical energy and catalyze an attachment of an inorganic phosphate group (Pi) to an ADP to form ATP.

ATP synthase has various components to its transmembrane protein structure: the rotor, the stator, the knob, and the rod. The middle, rotor component that allows for the passage of H+ ions. The stator component that is anchored in the inner mitochondrial membrane to hold the entire structure in place. The knob component that is the catalytic site for the attachment of Pi to ADP. Finally, the rod component that connects the rotor to the knob.

As the H+ ions travel, downhill its gradient, through the ATP synthase’s core, or the rotor rotates clockwise. As the rotor spins, it rotates the rod component. This rotation, activates a catalytic site in the knob where the ADP and Pi are combined to form ATP.

Macronutrient breakdown: Krebs Cycle

Introduction

The energy that is still present in the pyruvic acid molecule (C3H4O3) can be further captured by the cell via the citric acid cycle. The citric acid cycle occurs in the mitochondrial matrix. The cycle is fueled by the pyruvic acid molecule and by the beta-oxidation of fatty acids. While the glycolytic pathway is exclusively carbohydrate oxidation, Krebs cycle can utilize the breakdown products of carbohydrates, fats, and proteins. Furthermore, citric acid cycle intermediates, keto acids, can be siphoned off to produce fatty acids and nonessential amino acids. Thus, Krebs cycle is a source of building materials and a pathway for oxidation of food fuels.

Krebs Cycle

Pyruvic acid must be transferred across the mitochondrial membrane via active transport because it is a charged molecule. Once inside, the pyruvic acid gets transformed into acetyl CoA via three steps.

Initially, the pyruvic acid gets decarboxylated. The removed carbon is released as carbon dioxide gas. This is the first time that CO2 is released during cellular respiration. Then, the 2-carbon fragment (acetic acid or CH3COOH or CH3CO2H) is oxidized and the hydrogen is picked up by the NAD+ coenzyme. Finally, the oxidized acetic acid is combined with coenzyme A (sulfur-containing enzyme that was derived from vitamin B5) to produce acetyl coenzyme A (or acetyl CoA). Now, acetyl CoA is ready to be completely broken down by the mitochondrial enzymes.

Coenzyme A shuttles the acetic acid to an enzyme that condenses it with oxaloacetic acid (4-carbon acid) to produce the first substrate of the Krebs Cycle: citric acid (6-carbon acid). Oxaloacetic acid is a crucial molecule that is always needed to be present to pickup the acetic acid. (Hence, why it is also called the pickup molecule.) It is during the association of oxaloacetic acid with acetic acid that initiates the citric acid cycle. The atoms of the citric acid are rearranged to produce various intermediate molecules, named keto acids. The acetic acid gets decarboxylated carbon by carbon and oxidized. Primarily, the resulting by-products are the reduced coenzymes NAD+ (NADH+H) and FAD (FADH2). At the end of the Krebs Cycle, the oxaloacetic acid gets regenerated.

Each pyruvic acid produces 3 CO2 molecules, 1 FADH2, and 4 NADH + H reduced coenzymes, and 1 ATP. (Don’t forget that the 5 reduced coenzymes equate to the removal of 10 hydrogen atoms. All-in-all, during the citric acid cycle there are 2 decarboxylations and 4 oxidations.) The products are the sum of the products from the citric acid cycle and from the creation of acetyl CoA. Independently, the citric acid cycle results in 2 CO2 molecules, 1ATP, 3 NADH + H, and 1 FADH2 reduced coenzymes.

The combined products of the glucose oxidation (via 2 pyruvic acid molecules): 6 CO2, 2 ATP, and ten reduced coenzymes. The reduced coenzymes must be oxidized, via the electron transport chain, in order for the glycolysis and the Krebs cycle to continue.

Macronutrient breakdown: Glycolysis

Macronutrient breakdown: Glycolysis

Introduction

As the glucose molecule becomes engulfed into a cell via facilitated diffusion, it is immediately phosphorylated into glucose-6-phosphate (G6P). The phosphorylation is an important step in order to effectively trap and retain glucose within the cell. Most body cells lack the necessary enzyme to undo the phosphorylation. G6P is a different molecule when compared to a simple glucose. Thus, the addition of a phosphate group (PO4) maintains the intracellular concentration of glucose at a low level to ensure glucose can continue to “freely” enter the cell along its gradient. (Kidney tubule cells, liver cells, and intestinal mucosa cells have the necessary enzymes to undo the initial glucose phosphorylation. Hence, the importance of those cells in the glucose uptake and release mechanisms.)

Glycolysis

The array of catabolic and anabolic reactions of carbohydrates begin with glucose-6-phosphate. Glycolysis, or as it is sometimes referred as glycolytic pathway, occurs in the cytosol of cells. The series of steps produces two pyruvic acid molecules (CH3COCOOH). Initially, there is an investment of ATP molecules to transform glucose and perform an additional phosphorylation to produce fructose biphosphate. Fructose biphosphate is cleaved to produce two 3-carbon fragments. Then, each 3-carbon segments is oxidized. The removed hydrogen is, then, picked up by the coenzyme NAD+. (In other words, NAD+ becomes reduced.) An inorganic phosphate group (Pi) gets attached to each of the oxidized fragments. Later, the inorganic phosphate group get cleaved and ADP captures it to produce ATP.

The final products of glycolysis are: 2 pyruvic acid molecules, 2 reduced NAD+ coenzymes (NADH + H), and 4 ATPs. However, due to the initial investment of 2 ATP molecules, each glucose molecule produces a NET gain of 2 ATPs. During the oxidation of glucose, 4 hydrogen atoms get carried away by NAD+ coenzymes to be utilized in the electron transport chain.

Glycolysis can only be driven forward as long as there is free NAD+ to relieve glucose of its hydrogens. Oxygen is the required to oxidize the reduced NAD+ coenzyme. Oxygen, via the aid of the electron transport chain, relieves the hydrogen molecules to produce water in the mitochondrion. Whenever, there is an insufficient supply of oxygen then the reduced NAD+ coenzyme unloads its hydrogen onto the pyruvic acid molecule. The reduction of pyruvic acid via the 2 hydrogens transforms it into lactic acid. (In humans, the lactic acid is removed from the cells and is transported to the liver for further processing. The liver MAY convert lactic acid back to glucose-6-phosphate and then store it or release it to blood.) The cell can continue producing lactic acid and it can continue utilizing only glycolysis for its energy needs. However, glycolysis is not very efficient as it produces only 2 ATPs. The problem lies in tissue damage due to to the build up of lactic acid. The cells of the skeletal muscle can go on without tissue damage for the longest time. However, cells of the cardiac muscle have a shorter tolerance; and, ultimately, the cells of the brain have almost no tolerance to lactic acid. Once, oxygen becomes available then lactic acid is oxidized back to pyruvic acid. Pyruvic acid undergoes further changes and enters the citric acid cycle (Krebs Cycle), where it becomes further oxidized to produce ATP, CO2, and H2O.

So, to summarize, if there is no free NAD+, available, to carry the oxidized hydrogens then glycolysis cannot continue. We began with a glucose molecule (C6H12O6) and we ended up with a pyruvic acid molecule (C3H4O3). In the process, four hydrogen atoms were captured via the NAD+ coenzyme. Furthermore, pyruvic acid is a very important intermediate in the oxidation of a glucose molecule. It houses all of the energy from glucose that still needs to be further harvested with the aid of the citric acid cycle and, more importantly, the electron transport chain. Evidently, glycolysis produces only a NET gain of 2ATPs.

Thus, in order to capture the energy provided by the glucose molecule, pyruvic acid must travel from the cell’s cytosol into the mitochondrion to be further oxidized via the Krebs Cycle and the electron transport chain.