Content transcript :
Introduction to Metabolism
• Metabolism is the sum of an organism’s chemical reactions
• Metabolism is an emergent property of life that arises from interactions between molecules within the cell
A metabolic pathway begins with a specific molecule and ends with a product
The product of one reaction is substrate of the next
• Each step is catalyzed by a specific enzyme
CATABOLIC PATHWAY (CATABOLISM)
Release of energy by the breakdown of complex molecules to simpler compounds
EX: digestive enzymes break down food
ANABOLIC PATHWAY (ANABOLISM)
consumes energy to build complicated molecules from simpler ones
EX: linking amino acids to form proteins
Forms of Energy
• ENERGY = capacity to cause change
• Energy exists in various forms
(some of which can perform work)
• Energy can be converted from one form to another
KINETIC ENERGY –
energy associated with motion
– HEAT (thermal energy) is kinetic energy associated with random movement of atoms or molecules
POTENTIAL ENERGY = energy that matter possesses because of its location or structure
– CHEMICAL energy is potential energy available for release in a chemical reaction
THERMODYNAMICS
= the study of energy transformations
• CLOSED system (EX: liquid in a thermos)
= isolated from its surroundings
• OPEN system
energy + matter can be transferred between the system and its surroundings
• Organisms are open systems
The First Law of Thermodynamics
= energy of the universe is constant
– Energy can be transferred and transformed
– Energy cannot be created or destroyed
• The first law is also called the principle of CONSERVATION OF ENERGY
The Second Law of Thermodynamics
During every energy transfer or transformation
• entropy (disorder) of the universe INCREASES
• some energy is unusable, often lost as heat
Free-Energy Change (?G) can help tell which reactions will happen
?G = change in free energy
?H = change in total energy (enthalpy) or change ?S = entropy
T = temperature
?G = ?H - T?S
• Only processes with a negative ?G are spontaneous
• Spontaneous processes can be harnessed to perform work
Exergonic and Endergonic Reactions in Metabolism
• EXERGONIC reactions
(- ?G)
• Release energy
• are spontaneous
ENDERGONIC reactions
(+ ?G)
• Absorb energy from
their surroundings
• are non-spontaneous
Concept 8.3: ATP powers cellular work by coupling exergonic reactions to endergonic reactions
• A cell does three main kinds of work:
– Mechanical
– Transport
– Chemical
• In the cell, the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction
• Overall, the coupled reactions are exergonic
CATALYST = a chemical agent that speeds up a reaction without being consumed by the reaction
ENZYMES = biological catalysts
Most enzymes are PROTEINS
Exception = ribozymes (RNA)
ENZYMES LOWER ACTIVATION ENERGY BY:
– Orienting substrates correctly
– Straining substrate bonds
– Providing a favorable microenvironment
Enzymes change ACTIVATION ENERGY
but NOT energy of REACTANTS or PRODUCTS
• The REACTANT that an enzyme acts on
= SUBSTRATE
• Enzyme + substrate =
ENZYME-SUBSTRATE COMPLEX
• Region on the enzyme where the substrate binds = ACTIVE SITE
• Substrate held in active site by WEAK interactions (ie. hydrogen and ionic bonds)
TWO MODELS PROPOSED
• LOCK & KEY
Active site on enzyme
fits substrate exactly
• INDUCED FIT
Binding of substrate causes change
in active site so it fits substrate
more closely
– General environmental factors, such as temperature, pH, salt concentration, etc.
– Chemicals that specifically influence the enzyme
TEMPERATURE & ENZYME ACTIVITY
Each enzyme has an optimal temperature at which it can function (Usually near body temp)
COFACTORS
= non-protein enzyme helpers
• EX: Zinc, iron, copper
COENZYMES
= organic enzyme helpers
• Ex: vitamins
REGULATION OF ENZYME PATHWAYS
• GENE REGULATION
cell switches on or off the genes that code for specific enzymes
REGULATION OF ENZYME PATHWAYS
• FEEDBACK INHIBITION
end product of a pathway interacts with and
“turns off” an enzyme earlier in pathway
• prevents a cell from wasting chemical resources by synthesizing more product than is needed
NEGATIVE FEEDBACK
– An accumulation of an end product slows the process that produces that product
POSITIVE FEEDBACK (less common)
– The end product speeds up production
REGULATION OF ENZYME ACTIVITY
• ALLOSTERIC REGULATION
protein’s function at one site is affected by binding of a regulatory molecule at another site
• Allosteric regulation can inhibit or stimulate an enzyme’s activity
SOME ALLOSTERIC ENZYMES HAVE MULTIPLE SUBUNITS
• Each enzyme has active and inactive forms
• The binding of an
ACTIVATOR stabilizes
the active form
• The binding of an
INHIBITOR stabilizes
the inactive form
Enzyme Inhibitors
NONCOMPETITIVE inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective
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