Post on 04-Jan-2016
ENZYMES
What are enzymes?
Biological catalysts Most are proteins Some RNA
Regulate metabolism
Respond to changing needs of cell
All reactions require activation energy…
EA- needed to break existing bonds Even exergonic reactions require EA http://www.indiana.edu/~oso/animations/fire.html
Enzymes lower EA
Allow reactions to occur more quickly No effect on free-energy change http://www.stolaf.edu/people/giannini/flashanimat/enzymes/transition
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Enzymes reduce reliance on random collision of reactants
http://www.stolaf.edu/people/giannini/flashanimat/enzymes/prox-orien.swf
Enzyme-Substrate Complex
Every enzymes contains 1 or more active sites
Substrate binds – forms enzyme-substrate complex
Enzyme changes shape slightly – induced fit
http://www.stolaf.edu/people/giannini/flashanimat/enzymes/enzyme.swf
Shape of substrate may change also – straining existing bonds
Products form, diffuse away
Enzyme can be used again – is not used up in the reaction
Specificity of Enzymes
Most are highly specific due to shape of active site
May catalyze: Few closely related reactions Many only catalyze one reaction
Cofactors Some enzymes have 2 components
Apoenzyme – protein Cofactor – additional component Alone, neither is catalytic
Cofactors may be Inorganic – Ca, Mg, trace elements Coenzymes – organic, nonpeptide
Carrier molecules ATP Vitamins
Enzymes work best under specific conditions
Temperature In humans, most 35oC - 40oC Low temps –slow or not at all due to
molecular motion High temps – increase to a point;
denature
pH Humans, 6-8 Some, ex. stomach enzymes, work in
low pH Buffers – impt to minimize pH changes
Change in pH alters electric charges in R-groups of enzymes denature
Metabolic Pathways
Series of enzyme-catalyzed reactions
Enzymes aid in reaction coupling
Work in sequence Product of 1 enzyme, the reactant in
next
A enzyme 1> B enzyme 2> C
Each reaction is reversible, but Intermediate and
final products are removed
Drives the sequence of reactions to the right
Regulation of Enzyme Activity
Enzyme or Substrate Concentration
Feedback InhibitionAllosteric Enzymes
Regulation of Enzyme Activity
Synthesis of enzyme Gene turned on/off by signal
Substrate concentration If excess substrate, enzyme conc. is
limiting factor Low conc. of substrate can be rate-
limiting factor also
Feedback Inhibition Product of one enzymatic reaction
controls the activity of another
If product later in pathway inhibits an enzyme earlier in reaction sequence = feedback inhibition
Aenzyme 1>B enzyme 2>C enzyme 3>D enzyme 4>E
Allosteric Enzymes Exist in active and inactive forms Inactive form – active site doesn’t
allow substrate to bind http://www.stolaf.edu/people/giannini/flashanimat/enzymes/allosteric.swf
Allosteric site – receptor site, other than active site, which binds regulators and alters active site Inhibitors Activators
Effects of chemical agents:
Can destroy or inhibit enzymes Inhibition can be irreversible
Permanently inactivates enzyme Combines with active site, allosteric
site Ex. Mercury, lead, cyanide
Reversible inhibition – weak chemical bonds Competitive or noncompetitive
Competitive inhibition
Inhibitor competes with substrate for active site Structurally similar but cannot
substitute for substrate Temporarily occupies active site
Concentration matters
Noncompetitive Inhibition
Competitor binds to site other than active site alters shape
Useful in feedback inhibition
Allosteric inhibition is an example