Metabolic pathways form series of reactions that
regulate the concentration of substances within cells by
enzyme-mediated linear and circular sequences.
In biosynthetic pathways, small molecules are
assembled into large molecules. For example, simple sugars
are assembled into complex carbohydrates.
In degradative pathways, large molecules such as
carbohydrates, lipids, and proteins are broken down to form
products of lower energy. Released energy can be used for
cellular work.
Terms used in describing metabolic pathways include:
Substrates (= reactants ) are substances that enter
into a reaction.
Intermediates are substances that form between
the start and conclusion of a metabolic pathway.
End products are the substances present at the
conclusion of a reaction or pathway.
Energy carriers donate energy to substances by
transferring functional groups to them; ATP is the main type.
Enzymes are proteins that catalyze (speed up)
specific reactions.
Cofactors are organic molecules or metal ions that
assist enzymes or transport electrons/ atoms.
Transport proteins adjust the concentration
gradients at cell membranes in ways that influence the direction
of metabolic reactions.
Enzyme Structure and Function
Enzymes are proteins that serve as catalysts. They speed up
reactions.
Enzymes can be reused.
Enzyme actions are reversible.
Enzymes are selective and act upon specific substrates.
Enzyme-Substrate Interactions:Activation energy is the amount of energy needed to
bring colliding molecules to the transition state.
Enzymes increase the rate of a reaction by lowering the
activation energy through extensive bonding of substrate at the
active site. The active site is a crevice where the
substrate binds to the enzyme during a reaction according to the
induced-fit model. In order to proceed reactants must reach a
"transition" state.
Because enzymes operate best within defined temperature
ranges. High temperatures decrease reaction rate by disrupting
the bonds that maintain three-dimensional shape (denaturation
occurs). Most enzymes function best at a pH near 7; higher or
lower values disrupt enzyme shape and halt function. High
salinity also disrupts enzyme activity.
Some controls regulate the number of enzyme molecules
available by speeding up or slowing down their synthesis.
Feedback inhibition operates
when a substance triggers a cellular change that shuts down
production of that substance.
Allosteric enzymes have (in addition
to active sites) regulatory sites where control substances can
bind to alter enzyme activity. If this control substance is the end
product in the enzyme's metabolic pathway, feedback inhibition
occurs. Hormones are the signaling molecules in enzyme
control.
Coenzymes: Enzyme action often requires coenzyme
helpers such as NAD+, NADP+, and FAD. Ferrous iron is a
helper in cytochrome proteins involved in mitochondrial and
chloroplast electron transport systems.
REVIEW: Although it its too simple an explanation, the concept of a key fitting into a lock is
descriptive of the
REVIEW: During enzyme-catalyzed reactions, substrate is a synonym for
REVIEW: Enzymes
a. are very specific.
b. act as catalysts.
c. are organic molecules.
d. have special shapes that control their activities.
e. all of these
REVIEW: Enzymes increase the rate of a given reaction by lowering what kind of energy?
REVIEW: Allosteric inhibition is generally a result of
