MGD 4-5 Flashcards
what is activation energy?
minimum energy substrate must have to react
what is transition state?
high energy intermediate that lies between substrate and product (difference)
what increases the rate of a reaction?
temperature: increase no. of molecules with activation energy
concentration: increase chance of molecular collisions
what are enzymes?
biological catalysts that increase the rate of reaction by lowering the activation energy
facilitate formation of transition state
what are important features of enzymes?
highly specific, unchanged after the reaction, do not affect reaction equilibrium, increase rate of reaction, proteins, may require associated cofactors
what is the active site of an enzyme?
the place where substrates bind and where the chemical reaction occurs
formed by AA from different parts of primary sequence
what are features of active site?
are clefts or crevices (exclude water)
have a complementary shape to substrate - lock and key / induced fit (active site forms after binding of the substrate)
substrates are bound to enzymes by multiple weak covalent bonds
what is max reaction rate bound by?
number of enzyme active sites
reaches max velocity - retangular hyperbola (initially a lot of increase, then not so much)
what is the michaelis-menten model for enzyme catalysis?
a specific complex between the enzyme and the substrate is a necessary intermediate in catalysis
predicts a plot of velocity VS [S] is rectangular hyperbola
what is Vmax?
maximum rate when all enzyme active sites are saturated with substrate
what is Km
substrate concentration that gives 1/2 Vmax
what does a high and low Km mean?
low Km: high affinity for substrate (reaction rate quickly reached)
high Km: low affinity for substrate
Km of hexokinase and glucokinase - what causes the difference?
hexokinase low Km as always active - in all tissues
glucokinase high Km as ony in liver and only activated when glucose levels peak after feeding
what are enzyme inhibitors?
molecules that slow down or prevent an enzyme action
what are irreversible enzyme inhibitors?
bind v tightly, generally from covalent bonds
e.g. nerve hases e.g. sarin
what are reversible enzyme inhibitors?
non-covalent, can freely dissociate
can be competitive (bind at active site) or non-competitive (binds at another site on enzyme)
what are the 2 difference in terms of Km and Vmax between competitive and non-competitive reversible inhibitors?
competitive: affects Km (1/2 dissociation constant) but NOT Vmax - can be overcome by enough substrates
non-competitive: affects Vmax but not Km (curve doesn’t peak as high)
describe competitive inhibitor
resembles the substrate and binds to the active site, reducing the proportion of enzyme molecules bound to the substrate
what are short term regulations of enzyme activity?
- substrate and product concentration
- change in enzyme conformation:
a. allosteric regulation
b. covalent modification
c. proteolytic clevage
what are long term regulations for enzyme activity?
change in rate of protein synthesis
change in rate of protein degradation
explain how substrate and production concentration affects enzyme activity? what are isoenzymes?
substrate avaliability will affect the rate of enzyme activity
isoenzymes are different forms of the same enzyme that have different kinetic properties
what is product inhibition? example?
accumulation of product of a reaction inhibits the forward reaction
e.g. G-6-P inhibits hexokinase activity
what does an allosteric regulation graph look like?
shows a sigmoidal relationship between rate and substrate concentration, instead of rectangular hyperbola
multi subunit enzymes, can exist in 2 different conformations: T-state (low), R-state (high affinity)
shows sigmoidal because substrate binding to 1 subunit makes subsequent subunits bind easier (cooperative binding)
what is allosteric regulation? what are the different types and how do they work?
allosteric activators: increase the proportion of enzymes in R state
allosteric inhibitors: increase the proportion of enzymes in T state
bind somewhere else other than active site which changes the active site to which enhances the binding of the substrate, stabilising the high affinity R state
importance of allosteric regulation to phosphofructoinase (step 3 - irreversible)
PKF is allosterically regulated and sets the pace of glycolysis
activated by: AMP
inhibited by: ATP, citrate, H+
examples of covalent modification of enzymes through phosphorylation
- protein kinase - transfer terminal Pi from ATP to OH
2. protein phosphatases - reverse effects of kinase (remove P from porteins)
what does amplification by enzyme cascade achieve?
amplification of signals by kinase cascades allows amplification of the initial signal by several orders of magnitude within a few seconds
what is amplification by enzyme cascade?
when enzymes activate other enzymes, the number of affected molecules increases geometrically in an enzyme cascade
what are examples of enzymes activated by specific proteolytic cleavage?
- digestive enzymes - synthesised as zymogens in stomach and pancreas
- protein hormones e.g. insulin (synthesised as inactive precursors)
- blood clotting
- programmed cell death (apoptosis) - caspase (8)
proteolytic activation of chymotrypsinogen
chymotrypsinogen –(trypsin)–> chymotrypsin (pi)
chymotrypsin (pi) –(chymotrypsin)–> a chymotrypsin
from 1 inactive peptide to 3 active chains (a,b,c)
zymogen activation by proteolytic cleavage
by pancreatic protease
activation controlled by trypsin (from trypsinogen)
what are endogenous inhibitors of protease activity?
what happens without it?
- a1-antitrypsin
without it causes emphysema as proteases breakdown walls of elastase
what is a long term regulation of protease activity?
- change in rate of protein synthesis (enzyme induction/repression)
- change in rate of protein degradation (ubiquitin)
