Many of the smaller molecules of the cell, whether those used directly or those used as building blocks of macromolecules, are synthesized from precursors found among the intermediates of respiration. These anabolic pathways connect respiratory intermediates with other sugars, amino acids, fatty acids and fats, and nucleic acids. Virtually all the pathways in a cell are interconnected in a network of reactions.
Nitrogen is assimilated by cells by forming glutamate and glutamine from ammonia and ketoglutaric acid. In nitrogen-poor environments, energy must be expended. Rooted plants and algae can use ammonia, but primarily take nitrogen in as nitrate. Some microorganisms can take in nitrate or fix atmospheric nitrogen, but first convert these to ammonia. Other amino acids are then formed using the glutamate, possibly with other precursors, many of which are intermediates in respiration, such as pyruvate or oxaloace-tate. Formation of proteins from amino acids is discussed in Chapter 6.
Nucleotide synthesis begins with ribose for purines and glutamine for pyrimidines. The nitrogens are obtained from the amino acids glycine, glutamine, and aspartate. Fatty acids are formed using acetyl-CoA, similar to a reverse of the fatty acid oxidation described above. The process occurs in the cytoplasm and requires NADPH2, CO2, and Mn2+. Mammals can synthesize the saturated and monounsaturated fatty acids.
The biopolymers (starch, glycogen, protein, and nucleic acids) are produced by successive addition to the ends of the molecules. Often, ATP is converted to another nucleoside triphosphate for the reaction, such as uridine triphosphate, cytosine triphosphate, guani-dine triphosphate, or thymine triphosphate. The reaction usually splits the second phosphate bond, producing a monophosphate instead of a diphosphate. For example, if a monomer M is joined to a polymer of n units,
Then the resulting pyrophosphate (PPi) is hydrolyzed to orthophosphate. By eliminating the pyrophosphate reaction product, the first reaction is pushed toward completion by the mass action law.
Sulfur in the form of sulfide (S2~) is needed for the amino acid cysteine, which forms the disulfide bonds that are so important to protein folding. However, our aerobic environment provides sulfur mostly in the form of sulfate, SO42~. The sulfide can be formed anaerobically by microorganisms that use the sulfate as a terminal electron acceptor in the electron transport chain. Sulfate can also be reduced by a series of steps starting with ATP, followed by use of one NADPH2 to form sulfite (SO32~) and then three more NADPH2 to reduce the sulfite to sulfide.
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