Anabolism - Microbial metabolism
1. Photosynthesis
1.1 Light dependent reactions
a. Cyclic photophosphorylation
b. Noncyclic photophosphorylation
1.2 Light independent reactions
2. Other Anabolic Pathways
3. Integration and Regulation of
Metabolic Functions
Photosynthesis
• Photosynthesis is the single most important chemical
process on earth. It is the process by which plants use
solar energy to manufacture food.
• It changes light energy into food (chemical) energy.
• Photosynthesis sustains green plants and as a result all
other living things as well.
• Wood and fossil fuels — coal, oil and natural gas formed
from plants and animals that lived millions of years ago
— provide much of our electricity and heat.
• Green plants are the source of gasoline that we use to
power buses and cars.
• Fresh fruits, vegetables and grain, as well as meat from
animals that eat plants, give us the energy to work and
play and think
Chemicals and structures in
photosynthesis
• Photosynthesis is a process in which light energy is
captured by pigment molecules (called chlorophylls)
and transferred to ATP and metabolites.
• Photosystems, photosystem I (PS I) and photosystem II
(PS II), are networks of chlorophyll molecules and other
pigments held within a protein matrix in membranes
called thylakoids.
• Prokaryotic thylakoids are infoldings of the cytoplasmic
membrane
• Eukaryotic thylakoids are infoldings of the inner
membranes of chloroplasts.
• Stacks of thylakoids within chloroplasts are called grana.
Mechanism of photosynthesis
Light dependent reactions
• The light absorption and redox reactions of
photosynthesis are classified as lightdependent
reactions (light reactions) and light-independent
reactions (dark reactions).
• A reaction center chlorophyll is a special chlorophyll
molecule of photosystem I, which is excited by
transferred energy absorbed by pigment molecules
elsewhere in the photosystem.
• Excited electrons from the reaction center are passed to
an acceptor of an electron transport chain
• Protons are pumped across the membrane,
• A proton motive force is created, and ATP is generated in
a process called photophosphorylation which can be
either cyclic or noncyclic
Reaction center chlorophyll
Cyclic Photophosphorylation
• In cyclic photophosphorylation
electrons return to the original reaction
center chlorophyll after passing down the
electron transport chain.
• The resulting proton gradient produces
ATP by chemiosmosis.
Cyclic photophosphorylation
Noncyclic Photophosphorylation
• In noncyclic photophosphorylation,
photosystem II works with photosystem I, and
the electrons are used to reduce NADP+ to
NADPH.
• Therefore, in noncyclic photophosphorylation, a
cell must constantly replenish electrons to PS II.
• In oxygenic organisms, the electrons come from
H2O.
• In anoxygenic organisms, the electrons come
from inorganic compounds such as H2S.
Noncyclic photophosphorylation
Light-Independent Reactions
•
The light-independent reaction synthesize glucose from
carbon dioxide and water regardless of light conditions.
• ATP and NADPH from the light-dependent reactions
drive the synthesis of glucose in the light-independent
pathway of photosynthesis.
• The Calvin-Benson cycle of the light-independent
pathway occurs in three steps:
i. carbon fixation in which CO2 is reduced;
ii. reduction by NADPH to form molecules of G3P, which
join to form glucose;
iii. and regeneration of RuBP (ribulose 5-phosphate)
[also produced by PPP] to continue the cycle.
The Calvin-Benson cycle of the light-independent pathway
Other Anabolic Pathways
• Because anabolic reactions are synthesis
reactions, they require energy and metabolites,
both of which are often the products of catabolic
reactions.
• Amphibolic reactions are metabolic reactions
that can proceed toward catabolism or toward
anabolism depending on the needs of the cell.
• Examples are found in the biosynthesis of
carbohydrates, lipids, amino acids, and
nucleotides.
Carbohydrate Biosynthesis
• Gluconeogenesis refers to metabolic pathways that
produce sugars, starch, cellulose, glycogen,
peptidoglycan, etc. from noncarbohydrate precursors
such as amino acids, glycerol, and fatty acids
• It is highly endergonic requiring adequate supply of
energy.
• G3P from Calvin cycle, Acetyl-CoA, and
DHAP(dihydroxyacetone phosphate) from glycerol (from
fat) forms:
• (1)Fructose1,6,biphosphate --• (2)--fructose 6-phosphate (peptidoglycan) ---• (3)--glucose 6- phosphate (glycogen) ---• (4)--glucose (starch, cellulose)
Lipid Biosynthesis
• Lipids are diverse group of organic molecules
that function as energy-storage compounds and
as components of membranes e.g. Pigments
carotenoids in bacteria and plant photosystems
• Lipids are synthesized by a variety of routes.
• Steroids result from complex pathways involving
polymerizations and isomerizations of sugar and
amino acid metabolites.
• Fat is synthesized from glycerol and three
molecules of fatty acid—a reverse of the
catabolic reaction.
Steps in lipid biosynthesis
p.157 Bauman
• G3P (glyceraldehyde 3-phosphate)
obtained from Calvin-Benson cycle and
from glycolysis
• It converts into DHAP(dihydroxyacetone
phosphate), glycerol and fats
• Acetyl-CoA from glycolysis undergoes
reverse of beta-oxidation to form fatty
acids and fats
Amino Acid Biosynthesis
• Precursor metabolites for amino acid synthesis
are derived from glycolysis, Krebs cycle, PPP,
and other amino acids
• E. coli and most plants and algae synthesize all
their amino acids from precursor metabolites
(exception Lactobacillus)
• Humans need essential amino acids in diets
because they cannot synthesize amino acids
Precursor metabolites
Derived from glycolysis:
i. Glucose 6-phosphate synthesize lipopolysacharide
ii. Fructose 6-phosphate synthesize peptidoglycan
iii. Glyceraldehde 3-phosphate synthesize glycerol
portion of proteins
iv. Phosphoglyceric acid synthesize amino acids
v. Phosphoenolpyruvic acid (PEP) synthesize amino
acids, phenylalanine, trytophan, tyrosine
vi. Pyruvic acid synthesize amino acids, alanine,
leucine, valine
Precursor metabolites
Derived from Pentose phosphate pathway:
i. Ribose 5-P synthesize DNA, RNA, amino acid
and histidine
ii. Erythrose 5-P synthesize amino acids,
phenylalanine, trytophan, tyrosine
Derived from Krebs cycle:
i. Acetyl CoA synthesize portion of fatty acids
ii. Alfa-ketoglutaric acid synthesize amino
acids
iii. Succinyl –CoA synthesize heme
(cytochrome electron carrier)
iv. Oxaloacetate synthesize amino acids
Amino acid and protein synthesis
• Precursor metabolites are converted to amino
acids by an addition of amine group by:
• Amination when the amine group comes from
ammonia(NH3) e.g. Formation of aspartic acid
and the Krebs cycle intermediate oxaloacetic
acid
• Transamination, a reversible reaction in which
an amine group is transferred from one amino
acid to another by the action of enzymes
(transaminase) using coenzyme pyridoxal
phosphate derived from vitamin B6
• Ribosomes and ribozymes polymerize amino
acids into proteins
Nucleotide Biosynthesis
•
1.
2.
3.
4.
Nucleotides are produced from precursor
metabolites derived from:
glycolysis
the Krebs cycle: ribose and deoxyribose
from ribose-5 phosphate,
phosphate from ATP
purines and pyrimidines from the amino
acids glutamine and aspartic acid.
Nucleotide synthesis
Integration and Regulation of
Metabolic functions
• Energy released in catabolic reactions is used to
drive anabolic reactions.
• Catabolic pathways produce metabolites to use
as substrates for anabolic reactions.
• Cells use a variety of mechanisms to regulate
metabolism including control of gene
expression, which controls enzyme production
needed for metabolic pathways, and control of
metabolic expression in which the cells control
enzymes that have been produced.
Pathways of cellular metabolism
• The pathways of cellular metabolism can
be categorized into 3 groups:
• pathways synthesizing
macromolecules (proteins, nucleic acids,
polysaccharides, and lipids),
• intermediate pathways,
• and pathways that produce ATP and
precursor molecules (glycolysis, Krebs
cycle, pentose phosphate pathway, and
Entner-Doudoroff pathway).
Regulation of metabolism
Cells regulate metabolism by the following:
• Cells synthesize or degrade or channel and
transport proteins to regulate the concentration
of chemicals in the cytosols or organelles
• They synthesize enzymes needed to catabolize
a particular sustrate or stop the production of
beta-oxidation enzymes when there are no fatty
acids to catabolize
• If 2 energy sources are available, cells
catabolize the more energy efficient of the two
• They synthesize the metabolites they need and
cease synthesis if a nutrient is available as a
metabolite
Metabolism regulation
• Eukaryotic cells isolate particular enzymes for
different metabolic processess so as to avoid
their interference in the pathways
• Cells use inhibitory sites on enzymes to control
their activities
• Feedback inhibition slows or stops anabolic
pathways when product is in abundance
• The same substrate molecules used in catabolic
and anabolic pathways can be regulated by
using different coenzymes for each pathway e.g.
NADH is used almost exclusively with catabolic
enzymes, whereas NADPH is typically used for
anabolism