Energy Metabolism

Energy Metabolism Learning Goals

• The goals are to define metabolism and nutritional components, identify fundamental reactions, and understand energy transformation from food
• Also to describe metabolism changes after eating, between meals, and during fasting
• To identify vitamins/minerals involved in metabolism and their association to specific metabolic processes, and identify digestion/absorption challenges of folate and B12
• To discuss how to overcome nutritional challenges for nutrients in a vegan/vegetarian diet

Energy Metabolism - Metabolism

• Is defined as the sum of all chemical reactions in living cells
• It provides energy to cells for growth, repair, maintenance, and reproduction
• All organs, tissues, and cells participate
• The efficient process manufactures needed products and disposes of wastes
• Hormonal signals coordinate supply and demand
• Metabolic processes can be disturbed by disease, such as diabetes

Energy Metabolism - Fundamental Components

• Enzymes (proteins) mediate metabolic reactions
• Coenzymes (vitamins like niacin and riboflavin) enhance enzyme action
• Cofactors (minerals like iron and zinc) is required for enzyme activity
• Chemical reactions either require or release energy in the form of ATP

Energy Metabolism - Chemical Reactions

• Condensation: releases water and requires energy
• Hydrolysis: requires water and releases energy

The Body's Metabolic Work

• Metabolic chemical reactions in cells release heat; the body warm, and the rate of reactions helps maintain constant normal temperature
• Accelerated metabolism comes from stressors (burns, infection, surgery) and increases metabolism, fuels use at faster than normal rate and could cause fever, weight and lean tissue loss.
• Anabolism refers to energy-yielding nutrients used to build body compounds when not needed for energy
• Glucose units strung together to make glycogen chains, glycerol and fatty acids assembled into triglycerides and amino acids linked to proteins
• Anabolic reactions require energy provided by ATP
• Catabolism is the breaking down of body compounds when the body needs energy
• Glycogen is broken down to glucose, triglycerides broken down to fatty acids and glycerol and proteins broken down into amino acids
• Catabolic reactions release energy

Energy Use

• Energy manifests in heat, mechanical, electrical, and chemical forms
• Energy is stored in foods and in the body as chemical energy
• Energy-yielding nutrients broken into basic units and absorbed into the blood:
  • glucose from carbohydrates
  • glycerol and fatty acids from fat
  • amino acids from proteins

Adenosine Triphosphate (ATP)

• Is a high-energy compound with 3 phosphate groups: *Energy coin
• Bonds between phosphate groups described as readiness to release energy
• Transfers small amounts of usable energy to move muscles
• Supplies enzymes with energy to catalyze chemical reactions
• Produced continuously through breakdown of energy-yielding nutrients

The Chemical Pathways for ATP Production

• Glycolysis: (in cytoplasm, cytosol)
• TCA Cycle (tricarboxylic acid cycle) (Krebs or citric acid cycle) (in mitochondria)
• Electron transport chain (in mitochondria)

Glycolysis, Acetyl CoA, and TCA Cycle

• Glycolysis: Glucose (6-carbon) converted to pyruvate (3-carbon), produces 2 ATP, in cytosol (cytoplasm).
• Glucose carbons broken apart to produce pyruvate
• Hydrogen atoms attached to the carbons are transferred by coenzymes to the electron transport chain
• Pyruvate converts to acetyl CoA (2 carbon fragment and a coenzyme called CoA from the pantothenic acid, B5 vitamin) in mitochondria
• Fatty acids are broken down into 2-carbon fragments that combine with CoA to form acetyl CoA
• As carbons in fatty acids are broken apart to produce acetyl CoA, hydrogen atoms are released and transferred by coenzymes to the electron transport chain
• Glycerol can convert to pyruvate and then to acetyl CoA
• Amino acids can convert pyruvate then to glucose, OR to acetyl CoA and then they enter the TCA cycle
• In the TCA cycle (tricarboxylic acid cycle), enzymes break down acetyl CoA into carbon dioxide and hydrogen atoms
• Hydrogen atoms are carried by coenzymes to the electron transport chain
• 2 ATP molecules produced during the TCA cycle
• This process takes place in mitochondria

Electron Transport Chain

• In this last step in energy metabolism enzymes attach a phosphate group to ADP:
  • Max 34 ATP created by chemical energy from hydrogen atoms
  • Hydrogen atoms linked to oxygen produce water
  • Takes place in mitochondria
• Aerobic metabolism is defined by the Production of ATP via electron and requires oxygen in the final step
• Anaerobic metabolism, defined by production of ATP through glycolysis and does not require oxygen
• Complete oxidation results in up to 38 ATP

Glucose, Glycerol and Fatty Acids

• After glucose is produced it turns into pyruvate, then acetyl-CoA, then is used in the 2ATP TCA cycle, finally the 34ATP Electron Transport Chain, resulting in 38ATP
• Glycerol produced becomes pyruvate, then acetyl-CoA, to be used in the the TCA cycle/Electron Transport Chain
• Fatty acids can be converted in pyruvate, then acetyl-CoA, to eventually in the TCA cycle/Electron Transport Chain

The Role of Amino Acids

• In deamination, removal of amine group (NH2) from amino acid forms a keto acid; the amine group converts to ammonia (NH3), and ammonia is then convened into urea in liver and excreted by kidney - happens primarily in liver
• Amino acids can be
  • Converted to pyruvate and called glucogenic
  • Converted to acetyl-CoA and called ketogenic
  • Directly enter the TCA cycle

Glucose Production Sources

• Glucose produced from non-carbohydrate sources by gluconeogenesis in response to decreased glucose levels
• Any compound that can be converted to pyruvate can be used to make glucose, but compounds converted to acetyl CoA cannot
• Triglycerides consist of three fatty acids and a glycerol - that are broken down to to acetyl CoA
• Inefficient source of glucose
• Primary role of amino acids is to maintain body protein supply, after to be used as energy, must undergo deamination where it is converted to pyruvate
• Fairly efficient source of glucose when carbohydrate isn't available
• Glucose can be produced by glucogenic macronutrients, like amino acids and glycerol

Lipogenesis and Amino Acid Synthesis

• Lipids can be produced by glucose and ketogenic amino acids
• Amino acids can be produced through transamination, where an amino acid transfers amine group to keto acid, and a new amino acid and keto acid formed

Nutrient Energy and Production Associations

• Glucose: Yields energy and feeds into both glucose/fatty acid production, feeds into non-essential amino acid production when a source of nitrogen available, via pyruvate which is produced from glucose and glycerol, to make alanine
• Fatty acid: Yields energy and feeds into fatty acid production
• Glycerol: Yields energy and feeds into both glucose/fatty acid production and non-essential amino acid production when a source of nitrogen available, via pyruvate which is produced from glucose and glycerol, to make alanine.
• Amino Acid: Yields energy and feeds into both glucose/fatty acid production

Responses to Feasting and Fasting

• In feasting, consumption of more energy than is expended, and much of excess stored as body fat where after adequate liver and muscle glycogen stores, excess is burned for energy and displaces use of fat for allowing its accumulation - carbohydrates are then burned for energy while lipids add to body fat stores, and protein is used for growth and is burned
• During acute fasting liver and muscle glycogen are broken down and used to produce glucose fatty acids for energy from brain, nervous system, RBC and body tissue rest respectively
• Food scarcity leads to the body using carbohydrate/fat stores first (can last two day), before resorting to protein stores, ketone bodies will also supply the brain, nervous system, and rbc to release nitrogen & energy for other cells

Coenzymes

• Coenzymes assist in the production of energy and sometimes contain B vitamins as part of their structure, and are also converted to coenzymes.
• Protein Metabolism: niacin, folate, B6, B12
• CHO Metabolism : thiamin, riboflavin, niacin, panthothenic acid, B6 & B12
• Lipid metabolism : riboflavin, niacin, panthothenic acid and B12

B-Vitamins

• Involved in CHO metabolism -- Thiamin - good sources: pork, legumes, sunflower seeds, whole grain bread -Thiamine pyrophosphate (TPP), derived from thiamine by thiamine diphosphokinase, a CHO metabolism coenzyme
• Precursor of the coenzyme Flavin Adenine Dinucleotide (FAD) and Flavin Mononucleotide (FMN), involved in CHO and lipid metabolism, TCA cycle and ETC - dairy, meat, eggs, green vegetables, whole-grain breads
• Precursor for the synthesis of coenzymes NAD and NADP, involved in protein, CHO and lipid metabolism, TCA cycle and ETC, Amino acid tryptophan can be converted to niacin — meat, fish, peanut butter, whole-grain bread, certain vegetables (mushrooms)
• Pyridoxal phosphate (PLP) is the active form of B6 involved with protein/metabolism; protein-rich foods (poultry, meat, fish), certain fruits (bananas) and vegetables e.g. spinach
• Sources include beef liver, legumes (e.g. lentils), beets, leafy green vegetables, occurs naturally as folate in foods and as fortified folic acid in dietary supplements/fortified foods
• B12's sources include animal foods (red meat, dairy), and is fortified in some cereals

Vitamin B12 Absorption

• B12 binds to animal protein in the food
• It then re-binds to R protein (in salivary glands, binds to stomach)
• It is then released from R protein and binds to intrinsic factor in the small intestine (secreted by stomach)
• Intrinsic factor then releases B12 into intestinal cells
• Vitamin B12 deficiency is more common in vegans: it is derived almost exclusively from animal foods or fortified cereals
• Some have difficulty absorbing B12 and require monthly B12 injections

Pantothenic Acid, Biotin + Minerals

• Pantothenic acid is involved in CHO and lipid metabolism and is a key precursor for the biosynthesis of coenzyme A (CoA)
• It is everywhere and no danger of deficiency with a varied diet
• Biotin plays are role in the TCA Cycle and considered a coenzyme
• Roles in hormonal regulation and enzyme structure.

Oxidative Protection

• Free Radicals are unstable molecules that feature atoms with electrons that are NOT in pairs, they steal them from nearby atoms.
• This process leads to cell damage/death/fatigue/accelerated aging, sources can be endogenous or exogenous
• Reactive oxygen species are also free radicals but contain oxygen. This process leads to oxidative stress, or an imbalance between production of reactive species and antioxidant activity, can cause chronic disease

Antioxidants and Vitamins

• Antioxidants are substances that decrease the adverse effects of reactive species on normal physiological function by donating electrons to stabilize free radicals
• Vitamin E - alpha-tocopherol - biologically active form) is lipid-soluble and protects other substances from oxidation acting as an antioxidant
• Prevents oxidation of polyunsaturated fatty acids
• Vitamin E derived from vegetable oils, margarine, almonds, sunflower seeds, leafy green vegetables
• Water-soluble, protects water-soluble/fat-soluble compounds from oxidation by being oxidized itself, regenerates oxidized molecules
• Good sources of Vitamin C are from citrus fruits, red peppers, strawberries, broccoli
• Vitamin A aids vision, sexual reproduction, bone health, immune function, in the forms of Retinol, Retinal, and Retinoic acid
• Beta-carotene, sourced from Food such as plants with deep orange/dark green pigments, is effective antioxidant

Minerals and Redox Reactions

• Minerals act as cofactors to enzymes for converting free radicals to less damaging substances
• EX: manganese, zinc, copper in superoxide dismutase (SOD), iron in catalase, selenium in glutathione peroxidase
• Superoxide dismutase (SOD) - Converts superoxode radicals to hydrogen peroxide and oxygen
  • manganese for Mn-SOD from nuts, leafy greens
  • Copper and zinc for cytosolic/extracellular Cu-Zn-SOD from seafood, meats
• Catalase – Uses iron, converts hydrogen peroxide to water and oxygen
• Glutathione peroxidase – Uses selenium for hydrogen peroxide conversion