Enzymes Flashcards
define enzymes
enzymes are biological catalysts (mostly proteins) that speed up the rate of chemical reactions by providing an alternative pathway for the reaction to proceed and lowering its activation energy
what are the properties of enzymes?
effective in small amounts, since they remain chemically unchanged at the end of the reaction and can be reused
extremely efficient, proceeding at x10^(3-8) the rate of uncatalysed reactions
high degree of specificity regarding substrate molecules
can be denatured by heat / pH, have an optimum temperature / pH
regulated by activators and inhibitors
describe the structure of enzymes, and the four categories of amino acid residues
mostly globular proteins, with a specific three-dimensional conformation that must be maintained for them to remain functional
catalytic aa residues: directly involved in catalytic activity (forming or breaking of chemical bonds once substrate is bound)
binding aa residues: hold the substrate(s) in position via non-covalent bonds
structural aa residues: maintains specific 3D conformation of active site and enzyme
non-essential aa residues: no specific functions, removal or replacement does not affect enzyme’s function
what are the three types of cofactors and their functions?
additional non-protein component via covalent bonds or weak interactions to function, enzyme-cofactor complex is called a holoenzyme
inorganic metal ions: mostly small divalent ions (eg. Ca2+) binding reversibly, may either be active site component or affect activity through allosteric regulation (allosteric enzymes have multiple subunits and through conformational changes, bind activators of inhibitors at sites other than the active site)
coenzymes: loosely associates with enzyme, transient carriers of specific functional groups / hydrogen / electrons
prosthetic group: permanently and tightly bound
what are the types of enzymes, classified based on the types of reactions they catalyse?
oxidoreductase: redox
transferase: transfer of functional groups
hydrolases: hydrolysis
lyases: addition / removal of groups to form / undo double bonds
isomerases: transfer of groups within molecules to yield isomeric forms
ligases: formation of C-C, C-S, C-O, C-N bonds by condensation reactions
what is an effective collision?
when enzyme and substrate molecules collide in the correct orientation for the substrate to be bound to the enzyme’s active site, forming an enzyme-substrate complex
describe the mode of action of enzymes
after the formation of an enzyme-substrate complex, the substrate molecule is held in the active site by non-covalent bonds such as hydrogen and ionic bonds between the R groups of the binding aa and the substrate molecule
R groups of catalytic aa residues at active site catalyse the conversion of substrate to produce
this alteration in chemical conformation results in product being released from active site, since it is no longer complementary to active site structure
how do enzymes lower the activation energy of a reaction?
orienting substrates in close proximity in the correct orientation
straining critical bonds in the substrate so they attain unstable transition state
providing microenvironment that favours the reaction
how do enzymes lower the activation energy of a reaction?
orienting substrates in close proximity in the correct orientation
straining critical bonds in the substrate so they attain unstable transition state
providing microenvironment that favours the reaction
how do enzymes lower the activation energy of a reaction?
orienting substrates in close proximity in the correct orientation
straining critical bonds in the substrate so they attain unstable transition state
providing microenvironment that favours the reaction
why are enzymes specific?
due to the specific 3D conformation of the active site of each enzyme bc of the physical conformation and chemical properties (due to R groups) of binding aa at the active site, so only certain substrates with a complementary physical and chemical fit will enter active site
explain the two enzyme-substrate hypothesis
lock and key: 1894, Emil Fischer
exact fit / complementary shape or conformation between substrate and active site of enzyme (rigid structure)
explains substrate specificity
induced fit: 1959, Daniel Koshland
active site is flexible and not rigid, not in precise complementary conformation before binding, but changes / moulds active site, so more than one type of substrate can bind
what are the factors affecting the rate of an enzymatic reaction, and how?
substrate concentration, enzyme concentration, temperature, pH
concentrations: when low and limiting factor, increase results in proportional increase in ROR (not all active sites occupied, increase in frequency of collisions and effective collisions, more enzyme-substrate complexes formed per unit time, more product formed per unit time). when high, further increase does not cause increase in ROR (limited by e
what is the turnover number for an enzyme-catalysed reaction?
Kcat: maximum number of molecules of substrate that an enzyme can convert to product per catalytic site per unit time
(eg. one catalase molecule can convert approximately 5 million molecules of H2O2 to H2O and O2 per second)
how does temperature affect enzyme activity (below, at, and above optimum temperature)?
at low temperatures, enzymes are inactive
as temperature increases towards optimum, (quote data for temperature range), substrate and enzymes’ KE increases, more enzyme-substrate complexes formed and products produced per unit time, increased ROR
beyond the optimum temperature of (quote),
thermal agitation of enzyme disrupts hydrogen and ionic bonds, and other non-covalent interactions that stabilise its specific 3D conformation, loss in specific 3D conf of protein and its active site, no longer complementary fit with substrate, denatured, decreasing frequency of effective collisions (even though frequency of collisions increases), less enzyme-substrate complexes formed and products produced per unit time
