ENZYMES

What are enzymes? Biological catalysts Most are proteins Some RNA Regulate metabolism Respond to changing needs of cell

All reactions require activation energy… E A - needed to break existing bonds Even exergonic reactions require E A Enzymes lower E A Allow reactions to occur more quickly No effect on free-energy change Enzymes reduce reliance on random collision of reactants

Enzyme-Substrate Complex Every enzymes contains 1 or more active sites Substrate binds – forms enzyme- substrate complex Enzyme changes shape slightly – induced fit

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 35 o C - 40 o C 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 Inhibition Allosteric 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 A enzyme 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 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