Bioquímica e Fisiologia Microbianas

Licenciatura em Engenharia Química e Biológica

2º ano | 1º semestre | 2021/2022

T7
Metabolic Diversity

Organisms carry out a myriad of chemical

reactions, and the products of these reactions
are called metabolites.

Reactions are organized into biochemical

pathways.

Some are linear, in which the first molecule in the pathway is

often termed the starting molecule or the substrate of the
pathway. The last molecule is called the end product. Those
metabolites in between are called intermediates. An
intermediate of a pathway may be diverted from one pathway to
another pathway.

As long as the starting molecules or inputs of the pathway are

available and some end product is needed, these molecules will
flow into and out of the many pathways that function in cells.
Primary Metabolism

Living beings are capable of synthesizing a wide variety of chemical compounds essential to their
metabolism, which are called primary metabolites (Tropophase).

Primary metabolism includes:

→ Biosynthetic pathways of different macromolecules (carbohydrates, lipids and proteins) and their precursor
blocks (monosaccharides, fatty acids and amino acids).
→ Bioenergetic pathways leading to the formation of metabolic end-products.
Nutritional Types

Microrganisms can be grouped

nutritionally on the basis of
how they satisfy their
requirements for carbon,
energy, and electrons.

The energy, electrons, and

carbon that an organism
obtains from its environment
are used in chemical reactions
referred to as fueling reactions.
Nutritional Types
Nutritional Types

Only two sources of energy are

available to organisms: light and
chemicals.

Organisms that conserve energy from

chemicals are called chemotrophs, and
those that use organic chemicals are
called chemoorganotrophs.

Many Bacteria and Archaea can utilize

the energy available from the
oxidation of inorganic compounds, and
this form of catabolism is called
chemolithotrophy.

Phototrophs contain chlorophylls and

other pigments that convert light
energy into ATP and thus, unlike
chemotrophs, do not require
chemicals as a source of energy.
Nutritional Types
Nutritional Types

The cell, through the controlled transformation

of nutrients available in the extracellular
environment, obtains not only the biosynthetic
precursors but also the energy necessary for the
synthesis of its constituents.
Chemoorganoheterotrophs

Use organic compounds as sources of energy, electrons, and carbon.

Fungi, Protozoa, most Eubacteria and various Archaebacteria.

There are two general approaches a chemoorganotroph may use to catabolize its energy
source: respiration and fermentation.

Many chemoorganotrophs are used industrially to make foods (e.g., yogurt, pickles, cheese),
medical products (e.g., antibiotics), and beverages (e.g., beer and wine).
ATP

ATP is a high-energy molecule.

Cells carry out certain processes so that they can “earn” ATP, which they
“spend” in carrying out other processes.

In the cell’s economy, ATP serves as the link between exergonic reactions
and endergonic reactions.
ATP

The free energy delivered by substrate oxidation is sufficient to catalyze the phosphorylation reaction of ADP
to ATP.

Additionally, the cell, to ensure the synthesis of ATP, promotes the reoxidation of the involved coenzymes.

ATP production <-> Ability of the cell to maintain the balance between oxidation-reduction.
Oxidation-redution reactions

Energy production involves oxidation-

reduction reactions in which electrons are
transferred from the initial electron donor
(initial organic compound) which is oxidized,
to a terminal electron acceptor which in
turn is reduced.

Oxidation is associated with the transfer of

2 electrons and 2 H+.

The more electrons a molecule has and is

able to donate in a redox reaction, the more
energy rich the molecule is.

Glucose - can donate up to 24 electrons in

redox reactions - excelent source of energy.
Electron Carriers

Redox reactions are typically facilitated by

coenzymes that associate with the redox
enzymes that catalyze the reaction.
Chemoorganotrophic

Respiration is a form of aerobic or

anaerobic catabolism in which an
electron donor is oxidized with O2 (in
aerobic respiration) or some other
compound (in anaerobic respiration)
functioning as electron acceptors.

Fermentation is a form of anaerobic

catabolism in which organic
compounds both donate electrons
and accept electrons, and redox
balance is achieved without the need
for external electron acceptors.
Chemoorganotrophic

Most chemoorganotrophs use a wide

variety of organic molecules as energy
sources. They are degraded by pathways
that either generate glucose or
intermediates of the pathways used in
glucose catabolism.

Indeed, a common pathway often

degrades many similar molecules (e.g.,
several different sugars).

The existence of a few metabolic

pathways that each break down many
nutrientes greatly increases metabolic
efficiency by avoiding the need for a large
number of less metabolically flexible
pathways.
Chemoorganotrophic

Metabolism of organic compound

can be divided into 3 steps:

1. Conversion into simpler molecules

according to degradation pathways
– catabolic pathways – via enzymes
of degradation of polysaccharides,
lipases or proteases.

From carbohydrates, stage ends with

the formation of pyruvate (glycolytic
pathways),

Fatty acids from the cleavage of

triglycerides or phospholipids are
degraded to acetyl-coA through the β-
oxidation pathway.
Chemoorganotrophic

2. Metabolites, are channeled into the

Krebs cycle via acetyl-coA, leading to
the production of GTP, CO2 and H2O.

3. Coenzymes are reoxidized by the

transfer of electrons along an electron
transport chain associated with
membranes to oxygen which is thus
reduced to water.

Associated with the extrusion of

protons to the outer side of the
membrane and consequently leads to
the production of an electrochemical
gradient that is used by the enzyme
complex ATP synthase in the
phosphorylation of ADP and ATP.
Glycolytic pathways

Glycolytic pathways that mediate the degradation of glucose to pyruvate:

• Emben-Meyerhof-Parnas (Glycolysis - the most common pathway for glucose

degradation to pyruvate. It is found in all major groups of microorganisms, and
functions in the presence or absence of O2).

• Pentose phosphates (eukaryotes and bacteria use it to provide reducing power (as
NADPH) and important precursor metabolites for biosynthetic reactions)

• Entner-Doudorof (used by some Gram-negative bacteria, especially those found in

soil)

• Phosphoceltolase
• Methylglyoxal
Glycolysis
TCA cycle

TCA cycle enzymes are

widely distributed among
microrganisms.

In bacteria and archaea,

they are located in the
cytosol.

The complete cycle appears

to be functional in many
aerobic bacteria, archaea,
free-living protists, and
fungi.
Amphibolic pathways

Glycolysis and the Krebs cycle are

called central metabolic pathways
– they play a central role in
metabolism.

Amphibolic pathways (dual-

function pathways) → Function
simultaneously as catabolic and
anabolic pathways.
Electron Transport Chain
 ATP Yield

Bacterial ETCs are often shorter

and therefore transport fewer
protons across the plasma
membrane. Thus their ETCs have
lower ATP yields.
Electron Transport Chain

Because bacteria and archaea do not have mitochondria,

their ETCs are located primarily within the plasma
membrane.

Furthermore, some Gram-negative bacteria have ETC

carriers in the periplasmic space and even the outer
membrane.
Anaerobic respiration

Anaerobic respiration uses the

same three steps as aerobic
respiration.

Anaerobic respiration is the

chemoorganotrophic process whereby
a terminal electron acceptor other than
O2 is used for electron transport.

It is carried out by many bacteria and

archaea, and some eukaryotic
microbes.

The most common terminal electron

acceptors used during anaerobic
respiration are nitrate, sulfate, and CO2,
but metals and a few organic molecules
can also be reduced.
Anaerobic respiration

In both aerobic and anaerobic respiration processes,

NADH and FADH2 are used as the electron donors to
the ETC. However, for each NADH or FADH2 oxidized,
anaerobic respiration yields less ATP.

The lower ATP yield is due to the fact that all terminal electron
acceptors used during anaerobic respiration have less positive
reduction potentials than O2.

This results in a shorter ETC with fewer protons transported to

the periplasm per oxidation of NADH or FADH2 as compared
to aerobic respiration.

This explains why facultative anaerobes capable of both

aerobic and anaerobic respiration will use aerobic respiration
whenever possible.
Anaerobic respiration

An example of a facultative anaerobe

that can carry out both anaerobic
respiration and aerobic respiration is
Paracoccus denitrificans.

Under anoxic conditions (O2 absent), P.

denitrificans uses nitrate (NO3−) as its
electron acceptor, reducing it to
gaseous dinitrogen (N2), which is
released into the atmosphere -
Denitrification.

When this occurs in agricultural soils, it

depletes the soil nitrogen, which in
turn decreases crop yields.
Fermentation

• Some chemoorganotrophic microbes never respire and others may be unable to respire
under certain conditions due to the lack of ETCs.

• Those that can carry out aerobic respiration may repress the synthesis of ETC components
under anoxic conditions, making anaerobic respiration impossible.

For each of these microbes, NADH produced by the Embden-Meyerhof pathway reactions
during glycolysis must still be oxidized back to NAD+ even though there is no ETC to accept
electrons from NADH.

If NAD+ is not regenerated, the glycolysis will stop.

Many microorganisms use pyruvate as an electron acceptor for the reoxidation of NADH in a
fermentation process.
Fermentation