YSU BIOL 3702 - Chapter 10 - Metabolism: The Use of Energy in Biosynthesis

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BIOL 3702 Lecture OutlinePage 1 of 4Copyright © 2005 by Chester R. Cooper, Jr.Chapter 10 - Metabolism: The Use of Energy in BiosynthesisPrinciples of Biosynthesis Anabolism (= biosynthesis) Creation of more complex molecules from simpler ones, but requires energy input Carefully balanced with catabolic processes Energy required in terms of ATP is enormous Biosynthesis is shaped by several guiding principles: All macromolecules are derived from about 30 small precursors Many of the same enzymes used in both catabolism and anabolism End product regulation is generally more important in anabolic pathways To be efficient, anabolic pathways must operated irreversibly and are usually coupledwith the breakdown of ATP In eucaryotes, compartmentation permits different pathways to operate simultaneouslyand independently Anabolic and catabolic pathways typically use different co-factorsPhotosynthetic CO2 Fixation Most microbes can incorporate, or fix, CO2 via anaplerotic or other pathways Only autotrophs use CO2 as a sole carbon source and, therefore, utilize one of threedifferent reductive pathways to do so Energy for these pathways comes from photosynthesis or lithotrophy Three pathways to fix CO2 Calvin Cycle (reductive pentose phosphate cycle) Present in photosynthetic eucaryotes and most photosynthetic procaryotes Absent in Archaea, some procaryotic obligate anaerobes and microaerophiles Reductive Tricarboxylic Acid Pathway - used by some Archaea Acetyl-CoA Pathway - used by acetogens, methanogens, and sulfate reducers Calvin Cycle Occurs in: Chloroplast stroma (eucaryotes) Carboxysomes (procaryotes)BIOL 3702 Lecture Outline Chapter 10Page 2 of 4Copyright © 2005 by Chester R. Cooper, Jr. Three phases Carboxylation CO2 is incorporated into ribulose 1,5-bisphosphate (RuBP; 5 carbon molecule) toform two molecules of 3-phosphoglycerate (PGA; 3 carbon molecules) Catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase Reduction Phase PGA is reduced to glyceraldehyde 3-phosphate Requires one molecule of ATP and one molecule of NADPH for this reductionprocess to occur Regeneration Phase RuBP is regenerated via a series of reactions that resemble the pentosephosphate pathway Also generates a number of different carbohydrate structures that can be usedelsewhere in the cell Total energy expenditure (provided by photosynthesis or chemotrophy): 2 ATP molecules 3 NADPH moleculesGluconeogenesis Non-photosynthetic microbes synthesize sugars from reduced organic molecules Synthesis of glucose using noncarbohydrate precursors is termed gluconeogenesis Gluconeogenic pathway is remarkably similar to reverse glycolysis Gluconeogenic pathway Shares seven (7) enzymes with glycolysis Three steps are irreversible Some sugars are synthesized as nucleotide diphosphates which play a role inpolysaccharide formationAssimilation of P, S, and N Microbes require large amounts of phosphorous (P), sulfur (S), and nitrogen (N) P, S, and N comprise various portions of organic molecules used by cells Each of these elements is assimilated (incorporated) into organic molecules using differentpathwaysBIOL 3702 Lecture Outline Chapter 10Page 3 of 4Copyright © 2005 by Chester R. Cooper, Jr. Phosphorous assimilation Common sources Inorganic phosphate Organic phosphate esters Inorganic phosphate is obtained through the formation of ATP by: Photophosphorylation Oxidative phosphorylation Substrate-level phosphorylation Organic phosphate is obtained by microbes from hydrolyzing organic phosphates esterswith enzymes termed phosphatases Sulfur assimilation Microbes obtain sulfur from two sources: Sulfur-containing amino acids (cysteine and methionine) Sulfate (SO4-) by assimilation sulfate reduction pathway - SO4- is converted to H2Swhich is then incorporated into cysteine Nitrogen is acquired by assimilation or fixation Assimilation reactions Ammonia (NH4-) assimilation Direct incorporation of NH4- into organic material to form amino acids Indirect formation of amino acids by transamination reactions Assimilatory nitrate (NO3-) reduction - NO3- is reduced to nitrite (NO2-), which is thenfurther reduced to NH4- and subsequently incorporated into amino acids Nitrogen fixation - atmospheric nitrogen (N2) is catalyzed by the enzyme nitrogenase in athree step reduction process to form NH3, which can be subsequently used to form aminoacidsAmino Acid Synthesis Amino acids arise from carbon skeletons that are intermediates in many common pathways,e.g., methionine is derived from oxaloacetate formed in the TCA cycle Such processes conserve cellular energy and materials Synthesis of amino acids tends to be highly regulatedAnaplerotic Reactions The demand for intermediates of pathways can lead to a depletion of these compounds,thus disrupting the particular metabolic process, e.g., use of oxaloacetate from the TCAcycle to make amino acidsBIOL 3702 Lecture Outline Chapter 10Page 4 of 4Copyright © 2005 by Chester R. Cooper, Jr. Microbes have developed reactions, termed anaplerotic reactions, to replace these lostintermediates Most microbes will replace TCA cycle intermediates by CO2 fixation - NOT the same as theCalvin Cycle or other pathways that fix CO2 to produce carbohydrates CO2 is added to another compound that forms an intermediate Functions as a replacement process, not for cellular growth Maintains the operation of the TCA cycle Some microbes use the glyoxylate pathway to maintain the operation of the TCA cycle aswell as to grow on two-carbon sources (e.g., acetate) Essentially a modified TCA cycle Two key enzymes: Isocitrate lyase Malate synthase Pathway is essential for the survival of some pathogenic microbes in vivoNucleotide Synthesis Purines and pyrimidines are cyclic nitrogenous bases that are crucial to cellular function Pyrimidines - cytosine, thymine, uracil Purines - adenine and guanine Pyrimidines and purines form nucleosides when combined with a ribose or deoxyribosesugar Nucleotides are nucleosides with one or more phosphate groups attached, e.g., ATP, ADP,etc. Purine biosynthesis Inosinic acid produced from ribose 5-phosphate (11-step sequence) involving thecofactor folic acid [folic acid production is inhibited by sulfonamide drugs] Inosinic


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YSU BIOL 3702 - Chapter 10 - Metabolism: The Use of Energy in Biosynthesis

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