Glycolysis, Link Reaction, and Krebs Cell Respiration part 3

Respiration Process in which organic molecules act as a fuel Main fuel molecule: Glucose Secondary sources of fuel: fatty acids, glycerol, amino acids Organic molecules broken don in a series of stages Release chemical potential energy Used to synthesize ATP Four stages of glucose break down: Glycolysis Link Reaction Krebs Cycle Oxidative Phosphorylation

Review Phosphorylation Conversion of an energy- rich, but not very reactive, molecule into one that is MUCH more reactive AND whose chemical potential energy can be released and trapped more efficiently

CoEnzymes a nonprotein compound required for an enzyme to be able to catalyze a reaction NOT a substrate Do not become part of the reaction bind with the protein molecule (apoenzyme) to form the active enzyme (holoenzyme) bind to the active site of the enzyme and participate in catalysis but are not considered substrates of the reaction function as intermediate carriers of electrons, specific atoms or functional groups that are transferred in the overall reaction Examples: NAD, NADP, FAD, CoEnzymeA

NAD (Nicotinamide adenine dinucleotide) Co-enzyme Carrier molecule 2 linked nucleotides, each with a ribose One nucleotide has adenine One had nictinamide ring (this accepts hydrogen ion and 2 electrons Becomes “reduced” because it carries hydrogen ions and electrons “reduced NAD” aka NADH Removal of hydrogens is called an oxidation reaction Molecule that picks up the hydrogens is “REDUCED”

Other important coenzymes: FAD and NADP NADP (nicotinamide adenine dinucleotide phosphate) Slightly different form of NAD in photosynthesis Has a phosphate group instead of a hydrogen on carbon 1 of the ribose rings Called NADP (reduced NADP is NADPH) FAD (flavin adenine dinucleotide) Similar in function of NAD Used in Krebs Made of two nucleotides one nucleotide containing ribose and adenine One nucleotide containing unusual structure involving a linear molecule ribitol (instead of ribose)

Glycolysis Means “splitting glucose” Starting point for respiration in aerobic AND anaerobic conditions Aerobic with oxygen Anaerobic no oxygen present Location: cytoplasm (cytosol) of cell Multi-step process Each step is catalyzed by a specific enzyme (IMPORTANT!!!) ONE six carbon molecule splits into TWO three carbon molecules called PYRUVATE Energy required to begin (2 ATPs) Energy released at end (4 ATPs) Net total: 2 ATPs In order for Glycolysis to continue, we MUST have steady supply of NAD

ATP ATP 2 ATP 2 H 2 ATP Glucose (Hexose) (6C) Fructose phosphate (6C) Fructose biphosphate (6C) 2 molecules of triose phosphate (3C) Intermediates 2 molecules of pyruvate (3C) ATP ATP 2 ATP 2 NAD 2 H 2 Reduced NAD 2 ATP

Glycolysis First stage Phosphorylation with ATP (substrate level phosphorylation) 1st ATP to glucose glucose phosphate  fructose phosphate 2nd ATP to fructose phosphate  Fructose bisphosphate Fructose bisphosphate immediately breaks up into TWO molecules of triose phosphate Inorganic phosphate added to each triose phosphate creates triose bisphosphates Second Stage Dehydrogenation A phosphate is removed from each triose bisphosphate--. Back to 2 triose phosphates Hydrogen is removed from each triose phosphate by NAD  2 reduced NAD molecules Reduced NAD travels to mitochondria to be used in oxidative phosphorylation Third Stage Dephosphorylation Phosphate from triose intermediates removed to yield TWO more ATP molecules Product: 2 molecules of pyruvate (contains A LOT of chemical potential energy) When oxygen available, some of energy will be released via Krebs cycle and oxidative phosphorylation

Pyruvate + CoA + NAD   acetyl CoA + CO2 + reduced NAD The LINK Reaction Pyruvate enters mitochondria via ACTIVE transport Through outer AND inner membranes of mitochondria into MATRIX In the matrix, pyruvate is: DECARBOXYLATED Carbon dioxide molecule removed Diffuses out of mitochondria DEHYDROGENATED Hydrogen is removed (via NAD) Combined with Coenzyme A Makes a molecule called ACETYL Coenzyme A

CoEnzyme A Complex molecule Composed of: nucleoside Adenine + Ribose Vitamin Pantothenic acid Acts as a carrier of acetyl groups to the Krebs cycle

Other sources of Acetyl CoEnzyme A Proteins Amino acids Fatty Acids Broken down in mitochondria in a cycle of reactions in which each turn of cycle shortens the fatty acid chain by a two-carbon acetyl group Each acetyl group can react with CoEnzyme A  Acetyl CoEnzyme A Acetyl CoEnzyme A can now enter Krebs Cycle Oxygen MUST be present In order to BURN fat (break of those acetyl groups in a fatty acid) you must conduct AEROBIC types of exercise

Krebs Cycle Citric Acid Cycle Tricarboxylic Acid Cycle (TCA) 1937, Hans Krebs Closed pathway of enzyme controlled reactions Location : Matrix of Mitochondria Most important aspect: the release of hydrogens (that go onto oxidative phosphorylation to make ATP) In: Acetyl CoEnzyme A Enters Out: Carbon Dioxide released Reduced NAD released Reduced FAD released ATP is made

Steps of Krebs Cycle Acetyl CoEnzyme A combines with four- carbon compound (OXALOACETATE)  six- carbon compound (CITRATE) Citrate is DECARBOXYLATED (carbon dioxide removed) then DEHYDROGENATED (hydrogen removed by NAD and FAD) OXALOACETATE regenerated at the end of the cycle so it can happen all over again Products: ONE turn of Krebs yields: TWO carbon dioxide molecules ONE reduced FAD THREE reduced NAD ONE molecule of ATP (via intermediate)