This document summarizes microbial metabolism. It describes how hundreds of reactions take place simultaneously in a living cell in an organized manner, collectively called metabolism. Metabolic pathways involve a series of enzymatic reactions to produce specific products. Degradative processes break down complex molecules into simpler ones, releasing energy, while biosynthetic reactions form complex molecules from simple precursors. Key stages of metabolism include breaking biomolecules into building blocks like monosaccharides, converting these into intermediates like pyruvate and acetyl-CoA, and the final oxidation of acetyl-CoA through the Krebs cycle. The document also discusses various metabolic pathways like glycolysis, the pentose phosphate pathway, and the Entner-Doudoroff pathway

1. Microbial Metabolism
by
N. Sannigrahi, Associate Professor,
Deptt. Of Botany,
Nistarini College, Purulia (W.B) India

2.  Hundreds of reactions simultaneously take place in a living
cell, in a well-organized and integrated manner. The entire
spectrum of chemical reactions, occurring in the living system,
is collectively referred to as metabolism. A metabolic pathway
(or metabolic map) constitutes a series of enzymatic reactions
to produce specific products. The term metabolite is applied to
a substrate or an intermediate or a product in the metabolic
reactions.
 The degradative processes concerned with the breakdown of
complex molecules to simpler ones, with a concomitant
release of energy. The biosynthetic reactions involving the
formation of complex molecules from simple precursors. A
clear demarcation between catabolism and anabolism is rather
difficult, since there are several intermediates common to both
the processes.

3.  1. Conversion of complex molecules into their building
blocks:
 Polysaccharides are broken down to monosaccharide’s, lipids
to free fatty acids and glycerol, and proteins to amino acids.
 2. Formation of simple intermediates:
 The building blocks produced in stage (1) are degraded to
simple intermediates such as pyruvate and acetyl CoA. These
intermediates are not readily identifiable as carbohydrates,
lipids or proteins. A small quantity of energy (as ATP) is
captured in stage 2.
 3. Final oxidation of acetyl CoA:
 Acetyl CoA is completely oxidized to CO2, liberating NADH
and FADH2 that finally get oxidized to release large quantity
of energy (as ATP).

4.  Krebs cycle (or Citric acid cycle/TCA cycle) is the common
metabolic pathway involved in the final oxidation of all
energy-rich molecules. This pathway accepts the carbon
compounds (pyruvate, Succinate etc.) derived from
carbohydrates, lipids or proteins.
 ANABOLISM: For the synthesis of a large variety of
complex molecules, the starting materials are relatively few.
These include pyruvate, acetyl CoA and the intermediates of
citric acid cycle. Besides the availability of precursors, the
anabolic reactions are dependent on the supply of energy (as
ATP or GTP) and reducing equivalents (as NADPH + H+).
 The anabolic and catabolic pathways are not reversible and
operate independently. As such, the metabolic pathways occur
in specific cellular locations (mitochondria, microsomes etc.)
and are controlled by different regulatory signals.

7.  Carbohydrate, the primary source of energy of most micro-
organisms especially glucose, a simple sugar,
 Polysaccharides like starch, glycogen, cellulose, lignin,
hemicelluloses etc break down by enzymes to form glucose,
 Glucose is broken down by redox reactions to form Co2, H20
and ATP.
 Most of the microorganisms breaks down glucose anaerobic
ally by fermentation,
 Cellular respiration in aerobic process takes place by
Glycolysis, Krebs cycle /TCA cycle/ citric acid cycle, Electron
transport Chain.
 All the processes are catalyzed by enzymes and other factors
like temperature. pH, etc.

9.  Glucose after break down to form 2 mols pyruvic acid by two
stages-Preparatory phase and pay off or energy conserving
phase
 Overall reactions:
 Glucose+ 2 NAD+ +2ADP +2ip=2 Pyruvic acid+
2ATP+2NADH +2H,
 From 2NADH, 6 mol's of ATPs are produced, the total ATPs
becomes 6+2=8,
 It is also called EMP pathway or Embdeon- Mayerhoof Parnas
pathway,
 Other pathway of the oxidation of glucose is PPP ( Hexose
Monophosphate Shunt) or Entner- Doudoroff (ED) pathway.

12.  Many bacteria like Bacillus subtilis, E.coli, Enterococcus
faecalis etc perform this process,
 Production of intermediate pentose for nucleic acid synthesis,
production of certain amino acids etc,
 One ATP molecules for each glucose molecule oxidized and
produces NADPH coenzymes,
 Glucose 6 phosphate is oxidized to Ribulose 5 phosphate in
the oxidation phase , in the non-oxidative phase , the pentose
phosphate is recycled to form glucose 6 phosphate,
 PPP acts upon the need of the cell. If the requirement of the
reducing power is more than it proceeds the formation of
NADPH but if pentose is required , it function in the direction
of pentose. If the cell requires instant energy, the PPP stops
and glycolysis and TCA proceed.

14.  Describes the pathway of enzyme catalyzed chemical
reactions to catabolize glucose to Pyruvic acid different from
EMP or PPP pathway,
 Proposed by M. Doudoroff and N.Entner in 1952
 It uses 6-phosphogluconate dehydratasde and 2-keto -3
deoxyphosphogluconate aldose to create pyruvic acid from
glucose,
 It has net production of 1 ATP, 1 NADH and 1 NADPH for
each glucose molecule it processed through,
 Mostly Gram(-) bacteria like Pseudomonas, Azetobacter,
Rhizobium, Agrobacterium, E.Coli, Xanthomonas etc utilize
this pathway,
 It is also seen in Gram(+) bacteria like Enterococcous faecalis
and in the Archaea group of prokaryotes,

16.  Part of the aerobic cellular respiration,
 It is dependent on O2 to oxidize the reduced coenzyme such as
NADH2 and FADH2 produced during this cycle to generate NAD+
and FAD+ to carry out the cycle,
 It links the carbohydrate, Nitrogen and fat metabolism,
 The Acetyl coA derived from glycolysis and accepted by OAA to
form the foundation of this cycle,
 The various acids in this cycle are citrate-isocytrate-alpha
ketoglutarate-Succinyl CoA-Succinate-Fumarate-Malatre--OAA,
 Occur in the matrix of mitochondria using a number of enzymes,
 One turn of Krebs cycle generates 2 CO2, 3NADH, 1 FADH2 and 1
ATP/GTP molecule. As one glucose molecule produces 2 Acetyl
CoA, the entire to be multiplied by 2.

18.  The final stage of aerobic cellular respiration,
 ETC consists of electron carriers to transport electrons by
oxidizing NADH, FADH2 and ultimately to electron
acceptors, oxygen,
 During the flow of electrons from higher redox potentials to
the oxygen ATP molecules are formed,
 The ETC are capable of oxidation and reduction,
 Three types of carriers- flavoproteins containing flavin;
cytochromes with heme prosthetic group and ubiquinones or
Coenzyme Q which are non-protein carriers,
 The ETS include Cyt b, Cyt C1, Cyt c, cyt a1, and Cyt a3.The
terminal electron carrier is known as terminal oxidase and
passes electron to oxygen. Bacteria show variations in their
ETC due to different nature of energy providing substrate.

19.  Anaerobic respiration takes place in absence of oxygen,
inorganic substances are final electron acceptor,
 In Pseudomonas and Bacillus, Nitrate ions as final electron
acceptor while Desulfo-vibrio use sulphate ion as final
electron acceptor to form hydrogen sulphide,
 Fermentation produce diverse products like Lactic acid (
Streptococcous), Ethanol ( Saccharomyces), Propionioc acid (
Propion bacterium), Butyric acid or Butanol( Clostridium)
Succinic acid ( E.coli) and Formic acid ( Enterobacter) etc. as
per the nature of the respiratory substrate.
 But whatever may be, the amount of energy production is very
minimum due to partial oxidation of the respiratory substrates.
 Lipid and proteins are broken down and interlinked with
Krebs cycle.

20.  In contrast to algae and higher plants which are oxygenic (i.e.,
they evolve O2 during photosynthesis and have two photo-
systems that act in tandem or series, the photosynthetic
bacteria are anoxygenic (i.e., they do not evolve O2 during
photosynthesis and have comparatively simple photo
transduction machinery with only one type of photosystem and
reaction centre.
 Purple bacteria have Type II Reaction Centre which passes
electrons through bacteriopheophytin (bacteriochlorophyll
lacking central Mg2+ ion) to a quinone. Green sulphur bacteria
have Type I Reaction Centre that passes electrons to an Fe-s
protein.

21.  Type I Reaction Centre (The Fe-S Reaction Centre):
 In green sulphur bacteria, P-840 constitutes the reaction centre
of the only one pigment system present. Contrary to the cyclic
photosynthetic electron transport of purple bacteria, the
photosynthetic electron transport in green sulphur bacteria
appears to involve both cyclic and non-clyclic routes
 Type II Reaction Centre (The Bacteriopheophytin –
Quinone Reaction Centre):
 In purple bacteria, P-870 constitutes the reaction centre of the
only one pigment system present. When P-870 (B. Chl.a)
receives a photon of light, it get excited (*). An electron with
extra energy is ejected from it which is immediately (within
pico seconds) captured by bacteriopheophytin a (B. Pheo).
 By means of Calvin cycle or Reductive Carboxylic acid cycle
or Carbon assimilation by organic compounds are followed.

22.  The microorganisms synthesize the higher carbohydrates,
lipids, proteins, nucleotides inside their cells from the
chemical compounds formed by the catabolic process and
absorbed from the environment in which they grow. In
addition, purine and pyrimidine biosynthesis are also taken
into account as a part of the biosynthetic pathway during this
anabolism.
 Microbial metabolic reactions are efficient and speedier,
 Due to higher to surface volume ratio, the greater absorption
and efficiency,
 Various organic and inorganic compounds as well as light to
generate energy,
 Different electron transport chains depending on the source of
ele4ctron donors.

24.  ACKNOWLEDGEMENT:
 Microbiology & Phycology by Mishra & Das
 Microbiology- Pelczar, Chan & krieg
 Brock Biology of Microorganisms
 A Textbook of Microbiology by Chakraborty,
 A textbook of Microbiology by Dubey & Maheswari
 Different web pages
 Google for free download of images
 Disclaimer:
 This has been prepared without any financial interests, free for
students.