Enzymes Flashcards
Proteolytic enzymes catalyze the hydrolysis of ______
peptide bonds
Most proteolytic enzymes also catalyze the hydrolysis of an _____
Ester bond
An inactive enzyme without its cofactor protein part of a holoenzyme
apoenzyme
Requires a tightly bound meta ion
Metalloenzymes
Consequence of Thiamine (B1) deficiency
Beriberi (weight loss, heart problems, neurological dysfunction)
Consequence of Riboflavin (B2) deficiency
Cheliosis and angular stomatitis (lesions of the mouth), dermatitis
Consequence of Pyridoxine (B6) deficiency
Depression, confusion, convulsions
Consequence of niacin deficiency
Pellagra
Consequence of Pantothenic acid deficiency
Hypertension
Consequence of Biotin deficiency
Rash about the eyebrows, muscle pain, fatigue (rare)
Consequence of folic acid deficiency
Anemia, neural-tube defects in development
Consequence of B12 deficiency
Anemia, pernicious anemia, methylmalonic acidosis
Roles in vision, growth, and reproduction
Vitamin A Deficiencies include: night blindness, cornea damage, damage to respiratory and GI tract
Antioxidant vitamin
Vitamin C (ascorbic acid) Deficiencies include: scurvy (swollen and bleeding gums, subdermal hemorrhaging); Vitamin E Deficiencies include: Inhibition of sperm production; lesions in muscles and nerves (rare)
Vitamin in charge of regulation of calcium and phosphate metabolism
Vitamin D Deficiencies include: Rickets (children): skeletal deformaties, impaired growth, Osteomalacia (adults): soft bending bones
Vitamin that regulates blood coagulation
Vitamin K Deficiencies include: subdermal hemorrhaging
Oxidoreductases
Act on many chemical groupings to add or remove hydrogen atoms
Transferases
Transfer functional groups between donor and acceptor molecules. Kinases are specialized transferases that regulate metabolism by transferring phosphate from ATP to other molecules
Hydrolases
Add water across a bond, hydrolyzing it
Lyases
Add water, ammonia or carbon dioxide across double bonds, or remove these groups to produce double bonds
Isomerases
Carry out many kinds of isomerization: L to D isomerization, mutase reactions (shifts of chemical groups) and others
Ligases
Catalyze reactions in which two chemical groups are joined or ligated using energy from ATP
Enzyme exhibit a very high degree of
specificity
1+ inorganic ions, such as Fe2+, Mg2+, Mn2+, or Zn2+
cofactor
complex organic or metalloorganic molecule that act as transient carriers of specific functional groups
coenzyme
coenzyme or metal ion that is very tightly or covalently bound to the enzyme protein
prosthetic group
complete catalytically active enzyme together with its bound coenzyme and/or metal ions
holoenzyme
Translocases
Movement of molecules or ions across membranes or their separation within membranes
The molecule that is bound to the active site and acted upon by the enzyme
substrate
difference between the ground state energy level and the transition state energy level
activation energy
catalysts _____the activation energy and ______ the reaction rate
lower, increase
under standard conditions K’eq=
[P]/[S]
K’eq has a ______ relationship to Delta G’
Inverse
for a unimolar reaction S –> P a rate equaction expresses the rate of reaction and V =
V=k[S] where V is the velocity of the reaction and [S] is the concentration of the substrate
first-order reaction = rate depends only on the
concentration of S
Second-order reactions V=
V=k[S1][S2]
covalent interactions between enzymes and substrate _____the activation energy
lower
barrier to reaction, Delta G (activation energy) includes:
the entropy of molecules in solution the solvation shell of hydrogen-bonded water that surrounds and stabilizes most biomolecules in aqueous solution the distortion of substrates that must occur in many reactions the need for proper alignment of catalytic functional groups on the enzyme
replacement of the solvation shell of structured water around the substrate with weak bonds between substrate and enzyme
desolvation replaces most or all hydrogen bonds between the substrate and water
mechanism by which the enzyme itself undergoes a conformational change when the substrate binds, induced by multiple weak interactions with the substrate
induced fit enhances catalytic properties
catalytic functional groups aid in the cleavage and formation of bonds by a variety of mechanisms:
general acid-base catalysis covalent catalysis metal ion catalysis
Metal Ion Catalysis
help orient the substrate for reaction stabilize charged reaction transition states mediate oxidation-reduction reactions by reversible changes in the metal ion’s oxidation state
initial transient period during which ES builds up
pre–steady state
period during which [ES] and other intermediates remain constant
steady state
the traditional analysis of reaction rates
steady-state kinetics
Michaelis-Menten equation
V0 = Vmax [S] /Km + [S]
where V0 is the initial velocity, Vmax is the maximum velocity, [S] is the initial substrate concentration, and Km is a constant called the Michaelis constant
Km = [S] when
when V0 = ½Vmax
Lineweaver-Burk equation
1/V0 = Km + [S]/Vmax[S]
for a two-step Michaelis-Menten mechanism
Vmax = k2[Et]
•the rate constant for the conversion of E + S to E + P
specificity constant
the first substrate is converted to product and dissociates before the second substrate binds
example: Ping-Pong, or double-displacement, mechanism
What nomenclature is this
Intersecting Lines Indicate the Formation of a
Ternary Complex
Parellel lines indicate a ____________ pathway
Ping-pong (Double-Displacement)
for an enzyme where product release,_________ , is rate-limiting
EP → E + P ; (k3 is rate-limiting):
types of reversible inhibition
- competitive inhibition
- uncompetitive inhibition
- mixed inhibition
- noncompetitive inhibition
competes with substrate for active site of an enzyme; type of reversible inhibition; Km increases but Vmax is unchanged
competitive inhibition
the Michaelis-Menten equation becomes
V0 = (Vmax[S]) /(αKm + [S])
where α = 1 + [I]/K1 and KI = ([E][I]) /[EI]
What inhibition is indicated?
Competitive Inhibition
lines intersect at the y axis
binds at a site distinct from the substrate active site; binds only to the ES comples; type of reversible inhibition; both Vmax and Km deceases
Uncompetitive Inhibitors
What type of inhibition is depicted here?
Uncompetitive Inhibition
lines are parallel
affects the Vmax but not the Km; special case of α = α′
Noncompetitive inhibition
bind covalently with or destroy a functional group on an enzyme that is essential for the enzyme’s activity, or form a highly stable noncovalent association
irreversible inhibitor
undergo the first few steps until it is converted into a compound that combines irreversibly with the enzyme
class of irreversible inhibitors
suicide inactivator = mechanism-based inactivators
stable molecules designed to resemble transition states; type of irreversible inhibitors; bind to an enzyme more tightly than does the substrate in
transition-state analogs
catalytic activity increases or decreases in response to certain signals
allows the cell to meet changing needs for energy and biomolecules
regulatory enzymes
Modulation of Regulatory Enzyme
- allosteric enzymes = function through reversible, noncovalent binding of regulatory compounds called allosteric modulators or allosteric effectors (small metabolites or cofactors)
- reversible covalent modification
- binding of separate regulatory proteins
- removal of peptide segments by proteolytic cleavage
regulation in which the substrate and modulator are identical
homotrophic
regulation in which the modulator is a molecule other than the substrate
heterotropic
catalyzes the formation of carbamoyl aspartate, an early step in pyrimidine biosynthesis:
aspartate transcarbamoylase (ATCase)
The Kinetic Properties of Allosteric Enzymes Diverge from Michaelis-Menten Behavior
plots of V0 versus [S] usually produce a sigmoid saturation curve, rather than a hyperbolic curve
[S]0.5 or K0.5 represents the [S] giving half-maximal velocity of the reaction
