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

%20state.swf

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