introduction to metabolism. metabolism the sum of the chemical changes that convert nutrients into...
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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 diversityAutotrophs 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 complexA 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 reactionsATP 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 e- acceptor, 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 bondsUses lyases