Prescott's Microbiology,4/e

		Microbiology, 4/e Prescott, Harley, Klein
		Instructor Resources

10 Metabolism: The Use of Energy in Biosynthesis

CHAPTER OVERVIEW

This chapter presents an overview of anabolism. It then focuses on the synthesis of carbohydrates, amino acids, purines and pyrimidines, and lipids. It also provides a description of the assimilation of carbon dioxide, phosphorus, sulfur, and nitrogen. The chapter then concludes with a discussion of the synthesis of peptidoglycan and bacterial cell walls.

CHAPTER OBJECTIVES

After reading this chapter you should be able to:

o discuss the use of energy to construct more complex molecules and structures from smaller, simpler precursors

o discuss the way that biosynthetic pathways are organized to conserve genetic storage space, biosynthetic raw materials, and energy

o discuss the way that autotrophs use ATP and NADPH to reduce carbon dioxide and incorporate it into organic material

o describe the assimilation of phosphorus, sulfur, and nitrogen

o discuss the use of the TCA cycle as an amphibolic pathway and the need for anaplerotic reactions to maintain adequate levels of TCA cycle intermediates

o discuss the synthesis of glucose (gluconeogenesis) as a reversal of glycolysis and the synthesis of fatty acids by a process quite different from the b-oxidation catabolic process

o describe in general terms the synthesis of peptidoglycan and the construction of new cell walls after the peptidoglycan repeat unit has been transported across the cell membrane

CHAPTER OUTLINE

I. Introduction

A. Anabolism-the creation of order by the synthesis of complex molecules from simpler ones with the input of energy

B. Turnover-the continual degradation and resynthesis of cellular constituents

C. The rate of biosynthesis is approximately balanced by that of catabolism

II. Principles Governing Biosynthesis

A. The synthesis of large complex molecules (macromolecules) from a limited number of simple structural units (monomers) saves much genetic storage capacity, biosynthetic raw material, and energy

B. The use of many of the same enzymes for both catabolism and anabolism saves additional materials and energy

C. Many enzymes participate in both catabolic and anabolic activities; however, some steps are catalyzed by two different enzymes: one catalyzes the reaction in the catabolic direction, while the other reverses the step, thus permitting independent regulation of catabolism and anabolism

D. Coupling some anabolic pathways with the breakdown of ATP (or other nucleoside triphosphates) drives the biosynthetic reaction to completion

E. In eucaryotic cells, anabolic and catabolic reactions involving the same constituents are frequently located in separate compartments for simultaneous but independent operation

F. Catabolic and anabolic pathways use different cofactors: catabolic oxidations produce NADH, which is a substrate for electron transport, while NADPH acts as a reductant for anabolic pathways

G. Large assemblies (e.g., ribosomes) form spontaneously from their macromolecular components by a process known as self-assembly

III. The Photosynthetic Fixation of Carbon Dioxide-Calvin Cycle

A. Consists of three phases

1. The carboxylation phase-the enzyme ribulose 1,5-bisphosphate carboxylase catalyzes the addition of carbon dioxide to ribulose 1,5-bisphosphate, forming two molecules of 3-phosphoglycerate

2. The reduction phase-3-phosphoglycerate is reduced to glyceraldehyde 3-phosphate

3. The regeneration phase-a series of reactions is used to regenerate ribulose 1,5-bisphosphate and to produce carbohydrates such as fructose and glucose

B. Energy expenditure-each carbon dioxide takes three ATP molecules and two NADPH molecules, thus the formation of a single glucose molecule requires six turns through the cycle with an expenditure of 18 ATP molecules and 12 NADPH molecules

C. Sugars formed in the Calvin cycle can then be used to synthesize other essential molecules

IV. Synthesis of Sugars and Polysaccharides

A. Heterotrophs synthesize glucose from noncarbohydrate precursors in a process called gluconeogenesis

B. Gluconeogenesis is a functional reversal of glycolysis

1. Shares seven enzymes that also catalyze glycolytic reactions in the reverse direction

2. Three steps cannot be directly reversed and therefore require separate enzymes or multi-enzyme systems

C. Fructose is synthesized as part of the pathway

D. Other sugars are manufactured from glucose or fructose, or phosphorylated derivatives of those sugars

E. Polysaccharide production requires the use of nucleoside diphosphate sugars as precursors and are therefore not direct reversals of polysaccharide catabolism

V. The Assimilation of Inorganic Phosphorus, Sulfur, and Nitrogen

A. Phosphorus assimilation

1. Inorganic phosphates are incorporated through the formation of ATP by photophosphorylation, oxidative phosphorylation, and substrate-level phosphorylation

2. Organic phosphates obtained from the surroundings are hydrolyzed to release inorganic phosphates by enzymes called phosphatases

B. Sulfur assimilation

1. Organic sulfur in the form of cysteine and methionine can be obtained from external sources

2. Inorganic sulfate must first be reduced by a process called assimilatory sulfate reduction before it can be incorporated into cysteine

C. Nitrogen assimilation

1. Incorporation of ammonia can be easily and directly accomplished through reductive amination, glutamate formation, transamination, or through the actions of the enzymes glutamine synthetase and glutamate synthase

2. Assimilatory nitrate reduction involves the reduction of nitrate to nitrite, then to hydroxylamine, and finally to ammonia, which can then be incorporated by the routes described

3. Nitrogen fixation involves the reduction of atmospheric nitrogen to ammonia; this is catalyzed by the enzyme nitrogenase, which is found in only a few species of bacteria; this requires a large energy expenditure consuming almost 20% of the ATP produced by the host plant

VI. The Synthesis of Amino Acids-involves the synthesis of carbon skeletons, often by complex routes, from acetyl-CoA,TCA cycle intermediates, glycolytic intermediates, and pentrose phosphate pathway intermediates and then the addition of nitrogen through transaminase reactions

VII. Anaplerotic Reactions-replace TCA cycle intermediates that have been used to provide carbon skeletons for biosynthetic reactions

A. Involve carbon dioxide fixation for most microorganisms

B. Not limited to autotrophic organisms

C. Heterotrophic organisms use certain dioxide fixation reactions only to replace TCA cycle intermediates and to maintain metabolic balance, not to supply the carbon atoms needed for cell growth

VIII. The Synthesis of Purines, Pyrimidines, and Nucleotides-these molecules are critical for all cells because they are used in the synthesis of ATP, several cofactors, RNA, and DNA

A. Purine biosynthesis-very complex pathway in which seven different molecules contribute parts to the final purine skeleton; it involves the cofactor folic acid

B. Pyrimidine biosynthesis-aspartic acid and carbamoyl phosphate form the initial pyrimidine product, which can then be converted to other pyrimidines

C. Purine and pyrimidine bases are then joined with pentose sugars to form nucleosides

D. Phosphorylation of nucleosides forms nucleotides

IX. Lipid Synthesis

A. Fatty acids are synthesized using the substrates acetyl-CoA and malonyl-CoA, the reductant NADPH, and a small protein called an acyl carrier protein to carry the growing fatty acid

B. Unsaturated fatty acids can be formed either aerobically or anaerobically

C. Triacylglycerols are formed from the reduction of dihydroxyacetone phosphate (a glycolytic pathway intermediate) to glycerol 3-phosphate, which then undergoes esterification with two fatty acids to form phosphatidic acid; this can then be used to produce triacylglycerol

D. Phospholipids are also produced from phosphatidic acid using a cytidine diphosphate (CDP) carrier

X. Peptidoglycan Synthesis-involves a complex 8-stage process that forms the peptidoglycan repeat unit, which is then attached to the growing peptidoglycan chain after being transported across the cytoplasmic membrane; crosslinks are then formed by transpeptidation

XI. Patterns of Cell Wall Formation

A. Autolysins carry out limited digestion of peptidoglycan, and provide acceptor ends for the addition of new peptidoglycan units

B. Some organisms, such as gram-positive cocci, have only one or a few growth zones, usually at the site of septum formation

C. Rod-shaped organisms, however, usually have growth sites scattered along the cylindrical portion of the bacteria, as well as at the site of septum formation

Back