METABOLISM

OBJECTIVESAt the end of these lectures the student should :-

1. Understand the meaning of and importance of a balanced diet2. Understand the concept of metabolic balance3. Recognise the meaning of anabolism and catabolism4. Have a basic understanding of the mechanisms of

carbohydrate breakdown and assembly glycolysis and gluconeogenesis, and the metabolic pathways associated with these processes.

5. Have a basic understanding of the mechanism for ATP production in the mitochondrion.

6. Understand at the basic level lipid and protein metabolism.

NUTRIENTSThese are substances in food used by the body for growth, maintenance, and repair. 6 categories :-1-3 Carbohydrates, lipids, and proteins the major nutrients and bulk of what we eat.4 & 5 Vitamins and minerals, though crucial for health, are only required in minute amounts. 6 Water, 60 % by volume of the food we eat, is also a major nutrient.

Most foods provide a combination of nutrients. A diet consisting of foods from each of the five main food groups - grains, fruits, vegetables, meats and fish, and milk products - normally provides all necessary nutrients. The ability of cells (particularly liver) to convert one type of molecule to another enables the body to use a wide range of the chemicals found in foods and to adjust to varying food intakes.

There are limits to this ability to interconvert between molecules. Some 45 molecules, called - essential nutrients, cannot be made by the body or interconverted and must be contained in the diet. As long as these essential nutrients are present, the body can synthesize the hundreds of other molecules required for life, growth and good health.

Carbohydrates Except for milk sugar (lactose) and small amounts of glycogen in meat, carbohydrates are derived from plants.

Monosaccharides and disaccharides (sugars) - come from fruits, sugar cane, sugar beet, honey, and milk. Polysaccharides (starch) are found in grains, legumes, and root vegetables. Cellulose, present in most vegetables, cannot be digested but provides roughage, or fibre, which increases the bulk of the stool and facilitates defaecation.

The monosaccharide glucose is the carbohydrate molecule that is ultimately used by body cells. Carbohydrate digestion also yields the monosaccharides fructose and galactose that are converted to glucose by the liver before entering the general circulation. Glucose is a major body fuel and used to make ATP.

Many cells can use fat as an energy source, however neurons and red blood cells are almost entirely dependent on glucose for their energy needs. Consequently a shortage of blood glucose can affect brain function and lead to neuronal death. Hence the need for blood glucose levels to be carefully regulated. Glucose excess is converted to glycogen or fat and stored.

Carbohydrates are also used in; the synthesis of nucleic acids (pentose sugars -fructose) and attachment to externally facing plasma membrane proteins and lipids (Ag, signal molecules).

Monosaccharides - the basic subunits of –CHO’s. Usually have a 5 or 6 carbon backbone that forms into a ring structure, e.g. glucose, fructose.

Dissacharides – dimers of monosacharides. Can have different monosaccharides e.g. sucrose, maltose. Broken down into monosaccharides for absorption.

Polysaccharides -polymers of monosaccharides. Can be chains of one monosaccharide (cellulose), different monosaccharides, branched (glycogen) or unbranched. Broken down into monosaccharides for absorption.

Lipids Dietary lipid is typically in the form of neutral fats - triglycerides or triacylglycerols. Saturated fats are present in animal products (meat and dairy products) and in a few plant products (coconut). Unsaturated fats are in seeds, nuts, and most vegetable oils. Cholesterol is found in egg yolk, meats, and milk products.

Fats are digested to fatty acids and monoglycerides and reconverted to triglycerides for transport in the lymph. Although the liver is able to convert one fatty acid to another, it is unable to synthesize linoleic acid, a fatty acid component of lecithin. Linoleic acid is thus an essential fatty acid that must be ingested. Linoleic acid is found in most vegetable oils.

Dietary fats are needed for several reasons :-

a) they help the body absorb fat-soluble vitaminsb) triglycerides are the major energy fuel of hepatocytes and

skeletal musclec) phospholipids are an integral component of myelin sheaths and

cellular membranesd) the fatty deposits of adipose tissue provide a protective cushion

around body organs, an insulating layer beneath the skin, and an store of energy fuel

e) regulatory molecules called prostaglandins, formed from linoleic acid via arachidonic acid play a role in smooth muscle contraction, control of blood pressure, and inflammation.

Unlike fats, cholesterol is not used for energy. It stabilises plasma membranes and is a precursor of bile salts, steroid hormones, and many other essential molecules.

Most fats have the general structure of a glycerol molecule with 3 fatty acid side chains attached. The side chains can be of different lengths, straight (saturated) or kinked (unsaturated).

Proteins Animal products contain proteins with the greater proportion of essential amino acids. Proteins in eggs, milk, and most meats meet all the body's amino acid requirements for tissue maintenance and growth. Legumes (beans and peas), nuts, and cereals are protein-rich but nutritionally incomplete because they are low in one or more of the essential amino acids. Leafy green vegetables are well balanced in all essential amino acids except methionine, but contain only small amounts of protein. Strict vegetarians must carefully plan their diets to obtain all the essential amino acids and prevent protein malnutrition. When ingested together, cereal grains and legumes provide all the essential amino acids. Proteins are important structural materials of the body comprising for example; keratin in skin, collagen and elastin in connective tissue, muscle protein. They also have functional roles as, enzymes and hormones.

Whether amino acids are used to synthesize new proteins or are burned for energy depends on a number of factors:-

The all-or-none rule – all amino acids needed to make a particular protein must be present in a cell at the same time and in sufficient quantity. If one is missing, the protein cannot be made. Because essential amino acids cannot be stored, those not used immediately to build proteins are oxidized for energy or converted to carbohydrates or fats. Adequacy of caloric intake - for protein synthesis the diet must provide sufficient carbohydrate or fat for ATP production. If insufficient is available dietary and tissue proteins are used for energy. Nitrogen balance - in healthy adults the rate of protein synthesis equals the rate of protein breakdown and loss. This homeostatic state is called the body's nitrogen balance. The body is in nitrogen balance when the amount of nitrogen ingested in proteins equals the amount excreted in urine and faeces.

PROTEINS are polymers of amino acids (of which there are 20) and must be broken down into their component AA’s for absorption.

R and R’ are side groups that differ for each AA

When AA’s are joined together to construct proteins H2O is released (hydrolysis). The protein will end up with an AMINO end (-NH2) and a CARBOXYL end (-COOH). Protein characteristics are dependent on the length of the chain, the component AA’s and importantly the bends and twists that the different AA’s induce, called secondary and tertiary structure.

Vitamins Are essential organic compounds but needed only in minute amounts. They usually function as coenzymes (or parts of coenzymes), which act along side other enzymes to carry out a particular chemical task. e.g. the B vitamins riboflavin and niacin act as coenzymes in the breakdown (oxidation) of glucose for energy.

Because they are essential they must be taken in via foods. The exceptions are Vitamin D which is made in the skin, and Vitamin K synthesized by intestinal bacteria. The body can convert beta – carotene (the orange pigment in carrots and other foods) to vitamin A. Vitamins are found in all major food groups, but no one food contains all the required vitamins. A balanced diet is required to ensure a full vitamin complement.

Vitamins are fat soluble or water soluble.

Water-soluble vitamins – B and C are absorbed along with water from the gastrointestinal tract (except for vitamin B12, which must bind to gastric intrinsic factor to be absorbed). Not stored and excess lost in urine

Fat-soluble vitamins - A, D, E, and K bind to ingested lipid and are absorbed along with their digestion products. Anything that interferes with fat absorption will affect uptake of fat-soluble vitamins. Except for Vitamin K, fat-soluble vitamins are stored in the body. Excesses may cause problems.

Minerals The body requires moderate amounts of particular minerals; calcium, phosphorus, potassium, sulphur, sodium, chlorine and magnesium. It also needs trace amounts of a number of others (F, Co, Cr, Cu, I, Fe, Mn, Se, Zn.

METABOLISMCells are chemical factories that break down organic molecules to obtain energy, typically in the form of ATP. These reactions take place in the mitochondrion and provide most of the energy needed by a typical cell.To carry out these metabolic reactions, cells need a good supply of oxygen and nutrients. Oxygen is absorbed at the lungs, other materials from the digestive tract and the cardiovascular system ensures effective delivery throughout the body. Energy released from breakdown supports growth, cell division, contraction, secretion, as well as a number of other special functions that vary from cell to cell and tissue to tissue. As a result the energy requirements of different tissues is highly variable. When supply exceeds demand nutrients are stored in specialised tissues – adipose tissue, glycogen deposits, where they remain until needed. .

The endocrine system along with the nervous system, adjusts and coordinates the metabolic activity of tissues and controls the use, storage and remobilization of nutrient reserves.

The term metabolism describes all of the chemical reactions that take place in an organism.

At the cellular level these chemical reactions provide the energy needed to maintain homeostasis and to perform essential functions such as :-1) the periodic breakdown and replacement of the organic

components of a cell2) growth and cell division3) special processes, such as secretion, contraction, and action

potential propagation.

Catabolism – describes the breakdown of organic substrates to release energy that is used to make ATP or other high-energy compounds.

Catabolism takes place in a series of steps, beginning in the cytoplasm, where enzymes break down large organic molecules into smaller fragments. E.g. carbohydrates are broken down to short carbon chains, triglycerides are split into fatty acids and glycerol, and proteins are broken down to individual amino acids. Relatively little ATP is produced during these preparatory steps However, the simple molecules formed can be absorbed and processed by mitochondria, and the mitochondrial steps release significant amounts of energy. As mitochondrial enzymes break the covalent bonds that hold these molecules together, they capture roughly 40 percent of the energy released. The captured energy is used to convert ADP to ATP. The other 60 percent escapes as heat that warms the interior of the cell and the surrounding tissues. The ATP produced in the mitochondrion provides energy to support anabolism (the synthesis of new organic molecules), as well as functions like cell movement, muscle contraction and active transport.

The molecules produced by the anabolic processes are used in a number of ways:-To perform structural maintenance or repairs - cells must expend

energy to carry out ongoing maintenance and repair (metabolic turnover).

To support growth – dividing cells increase in size and produce extra proteins and organelles.

To produce secretions - secretory cells must make their products To build nutrient reserves - cells constantly lay down reserved

when nutrient supply allows, carbohydrate typically as glycogen, lipid as triglyceride.

In general, when a cell has free access to carbohydrates, lipids, and amino acids, it will break down carbohydrates first, lipids as second choice and only use amino acids when other sources are unavailable.

Catabolic and Anabolic Reactions

CARBOHYDRATE METABOLISM Most cells generate ATP and other high-energy compounds by breaking down carbohydrates, especially glucose. A summary of the reactions can be given as :-

C6H12O6 + 6O2 6CO2 + 6 H2O

The breakdown occurs in a series of enzyme catalysed steps, some of which release energy that is used to convert ADP to ATP. The complete catabolism of one molecule of glucose provides a cell with a net gain of 36 molecules of ATP.

Although most of the actual ATP production occurs inside mitochondria, the early steps take place in the cytoplasm. These early steps do not require oxygen so are said to be anaerobic, these cytoplasmic steps are called GLYCOLYSIS. The steps that take place in the mitochondrion require oxygen and are aerobic. They are called aerobic metabolism, or cellular respiration.

GLYCOLYSISIs the name for the series of steps that breakdown the 6 carbon glucose molecule into 2 X the 3 carbon molecule pyruvic acid (which at the normal pH inside the cell is in the form of the negatively charged ion pyruvate).

For glycolysis to take place there must be present :-

(1) glucose molecules, (2) appropriate cytoplasmic enzymes(3) ATP and ADP(4) inorganic phosphates(5) NAD (nicotinamide adenine dinucleotide), a coenzyme that is

reduced to NADH as part of the breakdown process (the NAD effectively holds on the H+ and releases it later to contribute to ATP production in the mitochondrion).

Glycolysis begins when an enzyme phosphorylates (attaches a phosphate group to the glucose molecule to make glucose-6-phosphate.

This effectively “uses up” one ATP molecule, it is advantageous however because it :-

1) traps the glucose molecule within the cell, because phosphorylated glucose cannot cross the cell membrane

2) prepares the glucose molecule for further biochemical reactions.

A second phosphorylation occurs in the cytosol before the six-carbon chain is broken into two three-carbon pieces.

Energy benefits appear when the pieces are converted to pyruvate with production of ATP and NADH. The net reaction is:- Glucose + 2NAD + 2ADP + 2Pi 2Pyruvate + 2NADH + 2ATP

The “anaerobic” glycolysis reaction sequence provides a net gain of 2 ATP for each glucose molecule converted to 2 pyruvic acid molecules.

More energy can be gained from the further breakdown of pyruvate, however these reactions require oxygen and take place in the mitochondrion. Because the supply of NAD is limited, glycolysis can only continue if NADH is relieved of its extra H. When oxygen is available NADH delivers its burden of H atoms to the enzymes of the electron transport chain in the mitochondria, which add them to O2 to form water. If oxygen is not available (as might occur during strenuous exercise) NADH unloads its H back to pyruvic acid, reducing it to lactic acid which diffuses out of the cells and is transported to the liver. When oxygen is available the lactic acid is oxidized back to pyruvic acid and enters the aerobic pathways within the mitochondria to be completely oxidized to H2O and CO2.

The Krebs cycle, Tricarboxylic Cycle, Citric Acid cycle - is the next stage of breakdown.It takes place in the matrix of the mitochondrion and breaks down pyruvate produced during glycolysis or fatty acids produced by breakdown of fats.

The pyruvate is firstly converted into acetyl CoA by a 3 step process. Acetyl coA then enters the TCA cycle and a series of reactions takes place that results in the production of :-CO2 + H2O + ATP + reduced coenzymes NADH + FADH

The coenzymes then enter the final step in the production of ATP which is the electron transport chain.The energy contained in the reduced coenzymes (H) is used to move H+ across the mitochondrial membrane. Dissipation of the H+ gradient through an ATP synthase generates ATP from ADP + Pi.

The purpose of the electron transport chain is to take the H on the coenzymes (NADH and FADH) and combine them with oxygen in a controlled manner such that the energy released is gathered and then used to ATP from ADP + Pi.

The components are complex, but most are proteins bound to metal atoms (cofactors), flavins (contain flavin mononucleotide, FMN, derived from the vitamin riboflavin) and cytochromes (iron-containing pigments. In the steps of the chain electrons are passed from one cofactor to the next and hydrogen ions are moved across the membrane of the mitochondrion. Eventually H+ re-enters the matrix through an ATP synthase and ATP is formed and O2 reacts with H+ to form water.

Glycogenesis and Glycogenolysis Although most glucose is used to generate ATP, unlimited supply of glucose do not result in unlimited ATP, because cells cannot store large amounts of ATP. When more glucose is available than can be used, rising ATP inhibits glucose catabolism and initiates storage of glucose as either glycogen or fat. Because the body can store much more fat than glycogen, fats account for 80 - 85 % of stored energy.

Glycogenesis (glycogen storage) takes place typically in the liver and muscles. Reversal and glucose production occurs when required.

LIPID CATABOLISMFats are the body's most concentrated source of energy. The energy yield from fat catabolism is nearly twice that from either glucose or protein catabolism - 9 kcal.g-1. Fat in the blood (in the form of chylomicrons) is hydrolyzed by plasma enzymes to fatty acids and glycerol that are taken up by body cells and processed in various ways.

Neutral fats are routinely oxidized for energy, with the separate oxidation of their two components glycerol and fatty acid chains. Glycerol is converted to glyceraldehyde phosphate (a glycolysis intermediate) that can enter the TCA cycle. Glyceraldehyde is effectivelyl half a glucose molecule and will enable 18 ATP’s to be made. Fatty acid oxidation occurs in the mitochondrion. The fatty acid chain is broken into 2C acetic acid fragments, and reduction of coenzymes are reduced. The acetic acid molecule then binds to coenzyme A to form acetyl CoA and onward to the TCA cycle.

Lipogenesis Neutral fats are continuously turned over in adipose tissue. Glycerol and fatty acids not needed for energy are recombined into triglycerides and stored usually in subcutaneous tissue

Lipogenesis occurs when cellular ATP and glucose levels are high. Excess ATP leads to an accumulation of acetyl CoA and glyceraldehyde-PO 4 that are channeled into triglyceride synthesis pathways. Acetyl CoA molecules are condensed together, forming fatty acid chains that grow two carbons at a time (hence almost all fatty acids in the body contain an even number of carbon atoms). Glyceraldehyde-PO 4 is converted to glycerol, which condenses with the fatty acids to reform triglycerides.

Protein Metabolism Proteins like other molecules wear out and must be broken down and replaced. Amino acids gained from the diet and transported in the blood are taken up by cells by active transport processes and used to rebuild and replace these worn out proteins. If more protein is ingested than needed for anabolic purposes, they are oxidized for energy or converted to fat.

In order to be oxidized for energy amino acids must have their amine group (-NH2) removed. The resulting molecule is then converted to pyruvic acid or to one of the keto acid intermediates in the TCA cycle. The steps include:-

Transamination - the amine group is removed from the AA by reaction with ketoglutaric acid. The ketoglutaric acid becomes glutamic acid and the original AA becomes a keto acid (oxygen swapped for NH2).

Oxidative deamination - the amine group of glutamic acid is removed as ammonia (NH3) and ketoglutaric acid is regenerated. NH3 combines with CO2 to yield urea and water that is excreted in the urine.

Keto acid modification - the keto acid is then modified so it can enter the TCA.

Amino Acid Metabolism

Absorptive State Anabolism exceeds catabolism. Glucose is the major energy fuel. Dietary amino acids and fats are used to remake degraded body protein or fat, and small amounts are oxidized to provide ATP. Excess metabolites, regardless of source, are transformed to fat if not used for anabolism.

Insulin essentially directs all of the events of the absorptive state. Rising blood glucose act as a humoral stimulus for the beta cells of the pancreatic islets to secrete more insulin.

Insulin binds to membrane receptors of its target cells, activates carrier-mediated facilitated diffusion of glucose into the cells enhancing glucose oxidation for energy, stimulating its conversion to glycogen (in adipose tissue, to triglycerides) and promoting protein synthesis and inhibiting the liver enzymes that promote gluconeogenesis.

Postabsorptive State In the postabsorptive state, between meals when blood glucose levels are dropping, the aim is to maintain blood glucose levels within the homeostatic range (80 - 100 mg glucose/100 ml). Constant blood glucose is required to “feed” the brain. The events of the postabsorptive state either make glucose available to the blood or save glucose for the organs that need it most.

The sympathetic nervous system and several hormones interact to control events of the postabsorptive state. As a result regulation is much more complex than in the absorptive state where insulin rules.

A trigger for initiating postabsorptive events is the reduction in insulin release that occurs when blood glucose falls. Declining glucose levels stimulate the alpha cells of the pancreatic islets to release the insulin antagonist glucagon.

Glucagon promotes a rise in blood glucose levels, targetting the liver and adipose tissue. Hepatocytes accelerate glycogenolysis and gluconeogenesis. Adipose cells mobilize fat stores (lipolysis), releasing fatty acids and glycerol to the blood. Glucagon release is inhibited after the next meal or whenever blood glucose levels rise and insulin secretion begins again.