Introduction to Metabolism
Metabolism
 The sum of the chemical changes that convert
nutrients into energy and the chemically
complex products of cells
 Hundreds of enzyme reactions organized into
discrete pathways
 Substrates are transformed to products via
many specific intermediates
 Metabolic maps portray the reactions
A Common Set of Pathways
 Organisms show a marked similarity in their
major metabolic pathways
 Evidence that all life descended from a
common ancestral form
 There is also significant diversity
 Autotrophs use CO2; Heterotrophs use organic
carbon; Phototrophs use light; Chemotrophs
use Glc, inorganics use S and obtain chem
energy through food generated by
phototrophs.
The Sun is Energy for Life
Phototrophs use light to drive synthesis
of organic molecules
Heterotrophs use these as building
blocks
CO2, O2, and H2O are recycled
Metabolism
 Metabolism consists of catabolism and
anabolism
 Catabolism: degradative pathways
 Usually energy-yielding!
 “destructive metabolism”
 FUELS -> -> CO2 + H2O + useful energy
 Anabolism: biosynthetic pathways
 energy-requiring!
 “constructive metabolism”
 Useful energy + small molecules --> complex molecules
Organization in Pathways
 Pathways consist of sequential steps
 The enzymes may be:
 Separate
 Form a multienzyme complex
 A membrane-bound system
 New research indicates that multienzyme
complexes are more common than once
thought
Catabolism and Anabolism
 Catabolic pathways converge to a few end
products
 Anabolic pathways diverge to synthesize many
biomolecules
 Some pathways serve both in catabolism and
anabolism and are called amphibolic pathways
Comparing Pathways
Anabolic & catabolic pathways involving
the same product are not the same
Some steps may be common to both
Others must be different - to ensure that
each pathway is spontaneous
This also allows regulation mechanisms
to turn one pathway and the other off
METABOLIC REGULATION
Regulated by controlling:
1. Amounts of enzymes
2. Catalytic activities
3. Accessibility of substrates
Digestion of food polymers:
 enzyme-catalyzed hydrolysis
Glycolysis:
 glucose catabolism
 generate ATP without consuming oxygen (anaerobic)
Citric Acid Cycle:
 metabolism of acetyl-CoA derived from pyruvate, fatty
acids, and amino acids
 acetyl oxidized to CO2
 operates under aerobic conditions
 reduction of coenzymes NAD+ and FAD; energy used to
produce ATP
Oxidative phosphorylation:
 reduction of molecular oxygen by NADH and FADH2
 energy of reduced compounds used to pump protons
across a cell membrane
 potential energy of electrochemical gradient drives
phosphorylation of ADP to ATP
The ATP Cycle
 ATP is the energy currency of cells
 In phototrophs, light energy is transformed
into the chemical energy of ATP
 In heterotrophs, catabolism produces ATP,
which drives activities of cells
 ATP cycle carries energy from photosynthesis
or catabolism to the energy-requiring
processes of cells
Redox in Metabolism
 NAD+ collects electrons released in catabolism
 Catabolism is oxidative - substrates lose
electrons, usually H- ions
 Anabolism is reductive - NADPH provides the
electrons for anabolic processes, and the
substrates gain electrons
WHY ATP?
Free energy is released when ATP is
hydrolyzed.
This energy drives reactions that need it
(eg. muscle contraction)
Recall coupled reactions
ATP has a higher phosphoryl transfer
potential
RECURRING MOTIFS IN METAB
Certain compounds keep on recurring or
appearing in metabolic reactions and
their functions are the same in the
processes
Metab looks complicated but reactions
are actually limited and repeating.
ACTIVATED CARRIERS
These species help carry out the
metabolic reactions, even nonfavorable
ones, at times
Example: ATP (activated carrier of
phosphoryl groups)
Activated carriers of electrons for
fuel oxidation: e- acceptors!
 Aerobic systems: O2 is the final eacceptor, but this does not occur
directly
 Fuels first transfer e- to carriers:
pyridine molecules or flavins.
NAD+:
nicotinamide
adenine
dinucleotide
Activated carriers of electrons for
fuel oxidation: e- acceptors!
FAD: Flavin
adenine
dinucleotide
Activated carrier of electrons for
reductive biosynthesis: e- donors!
NADPH: common
electron donor
R is phosphate
group
Activated carrier of carbon
fragments
COENZYME A: carrier of acyl groups
Activated carrier of two-carbon
fragments
VITAMINS
Many vitamins are "coenzymes" molecules that bring unusual chemistry
to the enzyme active site
Vitamins and coenzymes are classified
as "water-soluble" and "fat-soluble"
The water-soluble coenzymes exhibit the
most interesting chemistry
Key Reactions in Metabolism
1. REDOX reactions
Electron carriers are needed!
2. LIGATION reactions
 Bond formation facilitated by ATP cleavage
3. ISOMERIZATION reactions
4.GROUP TRANSFER
5.HYDROLYTIC reactions
Bond cleavage by addition of H2O
6.ADDITION of functional groups
to double bonds or REMOVAL of
groups to form double bonds
Uses lyases