Download CHAPTER 3 ESSENTIALS OF METABOLISM

CHAPTER 3
ESSENTIALS OF METABOLISM
Photo courtesy of Dr. Brian Oates
WHY IS THIS IMPORTANT?
• It is important to have a basic understanding of
metabolism because it governs the survival and
growth of microorganisms.
• The growth of microorganisms can have a
direct effect on infectious disease.
• Good metabolic function makes pathogens
more successful at causing disease.
OVERVIEW
BASIC CONCEPTS OF
METABOLISM
• Metabolism is:
– A series of chemical processes that go on in living
organisms.
– Used to obtain energy.
– Linked to growth.
BASIC CONCEPTS OF
METABOLISM
• Carbon and energy are required for growth.
• There are two processes by which carbon can be
obtained:
– Autotrophy – carbon is obtained from inorganic
substances (e.g. plants using CO2 to make sugar)
– Heterotrophy – carbon is obtained from other organic
molecules
• Nearly all infectious organisms are chemoheterotrophs.
• Chemoheterotrophs obtain energy by breaking
down other organic molecules and compounds.
OXIDATION & REDUCTION
REACTIONS
• Metabolism is broken
down into two parts:
– Catabolism – molecules
are broken down through
metabolic processes to
release the energy stored
in their chemical bonds.
– Anabolism – metabolic
processes in which the
energy derived from
catabolism is used to build
large organic molecules
from smaller ones.
OXIDATION & REDUCTION
REACTIONS
• Both anabolism and catabolism involve electron
transfer and oxidation and reduction reactions –
redox reaction.
• An oxidation reaction is a chemical reaction in
which an atom, ion or molecule loses one or more
electrons.
• A reduction reaction is a chemical reaction in which
an atom, ion or molecule gains one or more
electrons.
OXIDATION & REDUCTION
REACTIONS
• Oxidation and reduction reactions always occur
together.
– The combination of an oxidation reaction and a reduction
reaction are jointly referred to as redox reactions.
• When a substance is oxidized, it loses electrons.
• When a substance is reduced, it gains electrons.
RESPIRATION
• In metabolism, respiration occurs at the cellular
level and is not the same as breathing (respiration
at the macroscopic level).
• Cellular respiration describes catabolic processes
and is divided into:
– Aerobic respiration – metabolism that uses oxygen
– Anaerobic respiration– metabolism that does not use
oxygen
– Facultatively anaerobic respiration – metabolism that
can use oxygen but can also occur without it
METABOLIC PATHWAYS
• Metabolic reactions occur in series of chemical
reactions called pathways.
– The following is an example of a pathway. A is the
initial substrate and E is the final product of the
pathway, with B, C, and D being intermediates.
A
B
C
D
E
• Each step in the pathway is mediated or
facilitated by a specific enzyme.
ENZYMES
• Enzymes are proteins that act as catalysts for metabolic
reactions, making the reaction go faster.
• Enzymes work by lowering the energy of activation.
• Each enzyme is specific for a reaction.
•Enzymes are found in
all living organisms and
most cells contain
hundreds of types which
are constantly being
manufactured and
replaced.
PROPERTIES OF ENZYMES
• Enzymes have specific three
dimensional shapes: if the shape
changes, activity is inhibited.
• The shape of the molecule
provides a distinctive site called
the active site. It is here that:
– The substrate fits into the
enzyme’s active site.
– The enzyme and substrate
interact to form the enzymesubstrate complex.
• The active site has to have the
proper shape for the enzyme to
work.
PROPERTIES OF ENZYMES
• Enzymes are generally highly specific.
• A given enzyme catalyzes only one type of
reaction.
• Most enzymes react with only one particular
substrate.
• The shape and electrical charges found at the
active site allow for the reaction to work and
are responsible for the enzyme’s specificity.
PROPERTIES OF ENZYMES
• Some enzymes work on more than one
substrate but in these cases the enzymes
always work in a particular type of reaction.
– A proteolytic enzyme always degrades proteins
because it reacts only with peptide bonds.
COENZYMES AND CO-FACTORS
• Many enzymes can catalyze a reaction
only if other substances are present at
the active site.
– These enzymes are referred to as
apoenzymes.
• Co-factors are helper substances that
are inorganic ions such as magnesium,
zinc, or manganese.
• Coenzymes are helper substances that
are non-protein organic molecules.
• Co-factors or coenzymes bind to the
active site and change the shape of the
active site so the substrate now fits.
COENZYMES AND CO-FACTORS
• Coenzymes and co-factors can also be used as
carrier molecules.
– When a carrier molecule receives either electrons
or hydrogen atoms, it becomes reduced.
– When a carrier molecule releases electrons or
hydrogen atoms, it becomes oxidized.
COENZYMES AND CO-FACTORS
• Two coenzyme carrier molecules frequently
encountered in biological reactions are:
– NAD+ = nicotinamide adenine dinucleotide
– FAD = flavin adenine dinucleotide.
ENZYME INHIBITION
• Enzyme inhibition takes place in three ways:
– Competitive inhibition
– Allosteric inhibition
– Feedback inhibition
COMPETITIVE INHIBITION
• The inhibitor molecule is similar in structure to the substrate
and competes with the substrate to bind to the active site.
• When the inhibitor has bound to the active site, the substrate
cannot bind.
•The binding of the competitor is reversible and dependent
upon the relative numbers of inhibitor molecules and
substrate molecules present.
ALLOSTERIC INHIBITION
• Inhibitor molecules bind to a part of the enzyme away from the active
site: the allosteric site.
• This binding changes the shape of the active site in such a way that it
can no longer fit properly with the substrate.
• The binding of some allosteric inhibitors is reversible.
FEEDBACK INHIBITION
• The final product in a pathway accumulates and begins to bind
to and inactivate the enzyme that catalyzes the first reaction of
the pathway.
• It is reversible and, when the level of end product decreases, the
inhibition stops and the pathway begins to function again.
FACTORS THAT AFFECT ENZYME
REACTIONS
• Three major factors affect enzyme activity:
– Temperature – Can break hydrogen bonds and
change shape
– pH – Can break hydrogen bonds and change shape
– Concentration of substrate, product & enzyme –
Lower numbers of substrate, product, and enzyme
molecules means a lower level of activity.
CATABOLIC PROCESSES IN
METABOLISM
• Catabolic processes in
metabolism cause the
breakdown of large organic
molecules into smaller ones.
• These are called fueling
reactions because they
cause a release of energy.
CATABOLIC PROCESSES IN
METABOLISM
• There are three important pathways by which most
organisms release energy from nutrient molecules:
– Glycolysis
– Krebs cycle
– Electron transport chain
GLYCOLYSIS
• The catabolic pathway used by most organisms.
• The best example of this pathway is glucose
breakdown.
• The process itself is a series of chemical reactions.
Glucose
• Glycolysis occurs in
the cytoplasm and does
not require oxygen.
• Four ATP molecules
are produced in
glycolysis
– The first steps of
the pathway
consume two ATP
molecules.
– The net gain is two
ATP molecules.
4 – 2 = 2 ATP produced
Per glucose molecule (Net)
1 molecule of glucose
Produces:
– After a series of steps, the 6-carbon glucose molecule
2 ATP (net)
broken into two 3-carbon pyruvate molecules ->
2 reducedisNAD+
2 molecules
of Pyruvate
Krebs
cycle.
– NAD+ carries electrons to the electron transport chain.
• Glycolysis can lead to
further energy producing
pathways.
– Krebs cycle and cellular
respiration (aerobic)
– Fermentation (anaerobic)
THE KREBS CYCLE
• The Krebs cycle, A.K.A. the citric acid cycle or TriCarboxylic Acid cycle (TCA).
• It is an aerobic catabolic pathway seen in aerobic
cellular respiration.
• Pyruvate is further metabolized in this process.
• Pyruvate is oxidized to reduce NAD+ and modified
with coenzyme A to produce Acetyl-CoA complex.
THE KREBS CYCLE
• The Krebs cycle is a series of reactions in
which chemical changes occur.
– Within these reactions, hydrogen atoms are
removed and their electrons are transferred to
coenzyme carrier molecules.
– The hydrogen atoms are carried by NAD+ and
FAD to the electron transport system.
Three important things happen in the
Krebs cycle:
• Carbon is oxidized as CO2.
• Energy is captured and stored
when ADP is converted to ATP.
• Electrons are transferred to
coenzyme carrier molecules that
take the electrons to the
electron transport chain.
ELECTRON TRANSPORT CHAIN
• The electron transport chain is a sequence of
molecules.
– In eukaryotes, they are found in the inner
mitochondrial membrane.
– In prokaryotes, they are organized in the plasma
membrane.
ELECTRON TRANSPORT CHAIN
• Electrons are transferred to a final electron acceptor.
– In aerobic respiration, the final acceptor is oxygen.
– In anaerobic respiration, the final acceptor is an inorganic
oxygen-containing molecule.
CHEMIOSMOSIS
• As electrons move from one molecule to another in the chain, energy
is released via a process called chemiosmosis.
• As electrons are transferred along the electron transport chain, protons
are pumped out of the cell.
• This causes the proton concentration outside the cell to be higher than
inside the cell, causing a concentration gradient to form.
CHEMIOSMOSIS
• Specialized membrane proteins allow protons to re-enter the cell.
– Energy is released as protons re-enter the cell.
– This energy is used to bind phosphate to ADP, making the highenergy molecule ATP.
– The difference in proton concentration in this process is called the
proton motive force.
CHEMIOSMOSIS
• Cells using anaerobic respiration generate two
molecules of ATP from one glucose molecule.
• Cell using aerobic respiration generate thirty
eight total molecules of ATP from one glucose
molecule.
Aerobic respiration: 38 ATP / glucose molecule
Anaerobic respiration: 2 ATP / glucose molecule
FERMENTATION
• Fermentation is the enzymatic breakdown of carbohydrates
in which the final electron acceptor is an organic molecule.
•Different microorganisms use different
fermentation pathways.
ANABOLISM
• Anabolic reactions are classified as
biosynthetic reactions because they are used to
synthesize all the biological molecules needed
by the cells of living organisms.
• Biosynthetic reactions form the network of
pathways that produce the components
required by the cell for growth and survival.
• These reactions are fueled by the energy stored
in high-energy bonds in ATP.
ANABOLISM