Lecture 3 - Enzyme Kinetics III Flashcards
what inhibits pyruvate kinase and in what form of inhibition does it inhibit?
alanine non-competitively inhibits pyruvate kinase
what is alanine?
alanine = a negative allosteric modulator
alanine is one product of a series of enzyme-catalysed reactions, the first step of high is catalysed by phosphenol kinase
why does it make sense for the product of an enzymatic chain of reactions to inhibit one of the enzymes earlier in the chain?
?
what do many enzymes require in order to regulate activity?
many require Cofactors which regulate activity
Apoenzyme + Cofactor =
Holoenzyme
what do cofactors often derive from?
cofactors often derive from vitamins
what are cofactors referred to be when they are tightly bound to an enzyme?
prosthetic group
are cofactors specific to certain enzymes?
no, the same cofactor can be used by many forms of enzymes
what are the functions of vitamins: A, D, E & K?
A: roles in vision, growth & reproduction
D: regulation of calcium and phosphate metabolism
E: antioxidant
K: blood coagulation
ascorbate:
essentially an antioxidant that rescues the prolly hydorxylase enzyme by reducing the ferric ion
how does ascorbate function?
the enzyme prolly hydroxylase has tightly bound fe2+ ions within its structure which reacts with oxygen to form an oxidised iron complex (ferric ion) which inactivates the enzyme - to prevent this the ascorbate reduces the ferric ion
what does ascorbate deficiency result in?
scurvy
what do enzymes consist of?
multiple subunits and multiple active sites
cooperativity binding in enzymes:
enzymes can bind two substrate molecules at different binding sites, binding at one site affects the affinity for the other
(+) / ( - ) cooperativity:
• if the binding rate of the second substrate is increased, it’s called positive cooperativity
• if the binding rate of the second substrate is decreased, it’s called negative cooperativity.
is there association between the regulatory and active sites?
no, the regulatory site is separate from the active site
enzymes exist in two states:
tense and relaxed, binding of the effector influences further binding of substrate
“allosteric modulation” ???
hexokinase:
a highly regulated enzyme that catalyses [glucose + ATP → glucose-6-phosphate + ADP] at the beginning of glycolysis and its the rate limiting step
how does hexokinase work?
works by induced fit: a conformational change in the enzyme occurs upon binding to its substrate
also, allosterically inhibited by product
what mechanism does hexokinase catalyse?
the nucleophilic attack of phosphate by hydroxyl of glucose
hexokinase and glucokinase:
- conduct the same reaction, but have different properties
- hexokinase is found in the brain and in skeletal muscle, and is a regulatory enzyme (inhibited by high concentrations of its product), also has higher affinity for glucose
- glucoskinase is found in in the liver, and is absent in the brain and muscle, it is non-regulatory and has a lower affinity for glucose
- the difference in the two means that the organs (muscle and brain) that need the most energy are getting it and that the liver (that doesn’t need that much) doesn’t get wasted energy
where is -1K/M founds in an enzyme rate graph?
before the KM where the line passes the X-axis
what is Ki?
affinity of the enzyme for the inhibitor
