Chapter 12: Anabolism The Use of Energy in Biosynthesis

anabolism

the creation of order by the synthesis of complex molecules from simpler ones, requires the input of energy

principles governing biosynthetic metabolism

• coupling reactions with the breakdown of ATP drives anabolic pathways irreversibly in the direction of biosynthesis

• amphibolic is the reversal of catabolic pathways

  • many steps of the pathway are catalyzed by enzymes that participate in both catabolic and anabolic activities

  • some steps are catalyzed by two different enzymes

    • one in the catabolic direction and another in the biosynthetic direction

    • independent regulation of catabolism and anabolism

biosynthetic metabolism in eukaryotic cells

anabolic and catabolic reactions involving the same constituents are frequently located in separate compartments for simultaneous but independent operation

catabolic and anabolic pathways use different cofactors

• catabolic: use NADH for electron transport

• anabolic: use NADPH for electron transport

the use of many of the same enzymes for both catabolism and anabolism saves materials and energy

what does the synthesis of large, complex molecules help with?

from a limited number of simple structural units saves much genetic storage capacity, biosynthetic raw material, and energy

self assembly

process by once macromolecules have been made from simpler precursors, cell structures form spontaneously from the macromolecules

CO2 fixation in autotrophs

use CO2 as their sole or principal carbon source, requires much energy and reducing power

CO2 fixation: reductive pentose phosphate cycle

• widely used carbon fixation pathway

• consists of 3 phases that occur in the chloroplast stroma of eukaryotes and possibly in the carboxysomes of certain bacteria

  • carboxylation phase

  • reduction phase

  • regeneration phase

• incorporation of one CO2 uses 3 ATP and 2 NADPH

  • formation of 1 glucose requires 6 turn through the cycle with an expenditure of 18 ATP and 12 NADPH

  • sugars formed in the calvin cycle can then be used to synthesize other essential molecules

carboxylation phase

the enzyme ribulose 1,5-biphosphate carboxylase oxygenase catalyzes

• CO2 + ribulose 1,5-bisphosphate → (2) 3-phosphoglycerate

reduction phase

3-phosphoglycerate is reduced to glyceraldehyde 3-phosphate

reduction phase

series of reaction is used to regenerate ribulose 1,5-bisphosphate and to produce carbohydrates such as fructose and glucose

• similar to the pentose phosphate pathway and involves transkelolase and transaldolase reactions

synthesis of monosaccharides and polysaccharides

• gluconeogenesis

  • functional reversal of glycolysis: shares 7 enzymes with the glycolytic pathway, reversing their catabolic direction and uses 4 distinct enzymes or multi enzymes systems to catalyze steps that can’t be directly reversed

• once glucose and fructose are synthesized, other sugars are manufactured while attached to nucleoside diphosphate such as uridine diphosphate glucose (UDPG)

gluconeogenesis

• heterotrophs synthesize C6H12O6 from noncarbohydrate precursors

synthesis of peptidoglycan

• multistep process that involves 2 carriers:

  • uridine diphosphate and bactoprenol

• very vulnerable to disruption by antimicrobial agents, including antibiotics such as penicillin

• repeat unit is formed and is attached to the growing peptidoglycan chain after being transported across the cytoplasmic membrane

synthesis of amino acids

• nitrogen assimilation

• sulfur assimilation

• amino acid biosynthetic pathways

• anaplerotic reactions and amino acid biosynthesis

nitrogen assimilation

• ammonia incorporation

• many microorganisms use reductive amination to make alanine and glutamate → used as sources of amino groups

• amino groups are transferred from alanine or glutamate to other carbon skeletons by transamination reaction

sulfur assimilation

• organic sulfur (cysteine and methionine) can be obtained from external sources

• assimilatory SO4 reduction is used to reduce inorganic SO4 before making cysteine

amino acid biosynthetic pathways

• involves attachment of an amino group to a carbon skeleton

• carbon skeletons are derived from acetyl-CoA and from intermediates of the TCA cycle, glycolysis and the pentose phosphate pathway

anaplerotic reactions and amino acid biosynthesis

• biosynthetic functions of the TCA cycle are so important that many of its intermediates much be synthesized even when the TCA cycle is not functioning to catabolize pyruvate or to provide NADH for electron transport

• anaplerotic reactions replenish TCA cycle intermediates so that biosynthesis can occur

  • two major types of anaplerotic reactions have been observed

synthesis of purines, pyrimidines and nucleotides

• critical for all cells necessary for synthesis of ATP, several cofactors, RNA, and DNA → two types of bases are required:

  • purines (adenine and guanine)

  • pyrimidines (uracil, cytosine, and thymine)

• a nucleoside includes the base and sugar while a nucleotide also has the phosphate group

  • purine biosynthesis

  • pyrimidine biosynthesis

purine biosynthesis

• a complex pathway in which seven different molecules contribute parts to the final purine skeleton

• first purine product is the nucleotide inosinic acid → from which all other purine nucleotides can be made

pyrimidine biosynthesis

• starts with aspartic acid and carbamoyl phosphate forming the initial pyrimidine product (orotic acid) → can then be converted to pyrimidine nucleotides

lipid synthesis

• fatty acid synthesis catalyzed by fatty acid synthetase using substrates acetyl-CoA and malonyl-CoA

  • electron donor NADPH and a small protein called acyl carrier protein (ACP) → carries the growing fatty acid chain

  • fatty acid is lengthened by adding 2 carbons at a time to its carboxyl end

• triacylglycerols are formed from the reduction of dihydroxyacetone phosphate → to glycerol 3-phosphate

  • which undergoes esterification with two fatty acids to form phosphatidic acid → produce triacylglycerol

• phospholipids are produced from phosphatidic acid using cytidine diphosphate (CDP) carrier