H2 Biology 5 - Enzymes

Catalytic residues - catalyses chemical reaction

Contact residues - ensures specific conformation of active site(complementary to substrate)

Structural residues - maintain overall globular shape of molecule

Catalytic, contact residues make up active site - amino acids in 1^o are brought together by folding of chain in 2^o, predominantly 3^o structures

1. holding substrates closer together at correct angle, orientation at active site -> successful interaction & collision (anabolic)

2. strain bonds in substrates until they break(catabolic)

substrate is the "key" with shape complementary to enzyme's active site, enzyme is the "lock", enzyme's active site is the "keyhole".

when enzyme-substrate complex is formed, chances of successful reaction increase.

once products form, no longer fit into the active site & are released into medium; active site free to receive new substrate molecules.

initial shape of active site may not exactly be complementary to that of substrate

however, as substrate enters & binds to active site, it induces a conformational change of change in shape of active site -> substrate can fit even more snugly into active site -> allows better interaction between substrate and catalytic residues @ active site for successful reaction.

1. 3D conformation of active site is complementary to that of substrate, allows greater interaction b/w CG of substrate & catalytic residues of active site -> spatial fit

2. active site chemically compatible to substrate, capable of bonding with each other; substrate of enzyme depends on nature of amino acids making up active site -> chemical fit

1. pH

2. temperature

3. enzyme concentration

4. substrate

when investigating a given factor(iv), all other factors kept constant & optimum levels

@ low temps near/below 0^oC, enzyme activity low. enzyme inactivated, minimal KE for collision w/ substrate to form E-S complexes

as temp. increase before optimum temp, rate of reaction increases. increase in heat increases KE of substrate & enzyme, increases molecular motion. freq. of successful collisions b/w substrate, enzyme increases; rate of formation of E-S complexes increases -> rate of formation of products increase

@ optimum temp, max rate of reaction. highest frequency of successful collisions b/w substrate & enzyme.

as temp. increases beyond optimum temp, rate of reaction decreases. heat disrupts H bonds, hydrophobic interactions in 2^o, 3^o structures -> specific 3D conformation of AS lost -> enzyme denatured. substrate can no longer bind to AS of enzyme to form E-S complexes, decreasing rate of formation of E-S complexes -> rate of formation of products decreases.

@ optimum pH, max rate of reaction. bonds maintaining 2^o, 3^o of enzymes intact -> highest frequency of successful collisions b/w substrate & enzyme -> increases rate of formation of E-S complexes -> rate of formation of pdt. increases.

@ pH other than optimum, rate of reaction decreases. change in pH alters ionic charge of -NH3+ and -COO- R groups -> ionic, H bonds maintaining conf. of AS disrupted; loss of specific 3D conf of enzyme & AS(denatured) -> substrate cant bind to AS, rate of formation of E-S decreases, rate of formation of pdt decreases.

@ low conc, rate low. as conc increases, more AS available for successful collisions w/ substrates -> increase in frequency of successful collisions b/w substrate, enzyme -> rate of formation of E-S complexes increase, rate of formation of pdt increases(still LiFa)

@ very high conc, rate remains constant. increase in conc wont result in further increase in rate, no longer LiFa, substrate conc is LiFa

@ low conc, rate increases w/ increase in conc. many enzyme molecules have unoccupied AS(LiFa)

as conc increases, rate increases linearly. more substrates available for successful collisions b/w substrate, enzyme -> increase in frequency of successful collisions b/w enzyme, substrate -> rate of formation of pdt increases(still LiFa)

@ very high conc, as conc increases rate constant. all available AS saturated w/ substrates. enzyme conc now LiFa

compound w/ similar 3D conformation to substrate, can enter & bind to AS.

when bound to AS, prevents substrate from entering AS, prevents formation of E-S complexes & pdt as E-I complexes formed instead -> decreases rate of reaction.

@ low substrate conc, conc of substrate=conc of CI -> frequency of E-S collisions = frequency of E-I collisions. no. of E-S complexes = no. of E-I complexes, low rate.

@ high substrate conc, substrate outcompete CI for binding to AS, frequency of E-S collisions higher than frequency of E-I collisions -> more ES complexes than EI formed, higher rate.

@ very high substrate conc, substrates outcompete CI for binding to AS, allows max rate.

CI reversible, overcome by high substrate conc

compound w/ no structural similarity to substrate, binds to enzyme @ allosteric site, results in conformational change of enzyme, AS altered, substrate cant bind to AS.

formation of EI complexes prevents formation of ES complexes, prevents formation of pdt & rate decreases. effect not changed by substrate conc

can be reversible/irreversible