Lecture 7: Enzyme Regulatory Mechanisms Flashcards
General ways to regulate enzyme activity
- allosteric control
- multiple forms of enzymes
- reversible covalent modification
- proetolytic activation
- controlling the amount of enzyme present
allosteric control
- linked to noncompetitiv einhibition
- allosteric proteins contain distinct regulatory sites and multiple functional sites
–> binding of smal molecules at regulatory sites
- cooperativity
–> activity of one functional site affects the activity at others (information is transduced)
ex: aspartate transcarbamyolase
cooperativity
- allosteric control
- activity at one functional site affects activity at others
multiple forms of enzymes
- isozymes or isoenzymes
- isozymes provide an avenue for varying regulation of the same reaction at distinct locations or times to meet specific physiological needs
- homologous enzymes with a single organism that catalyze the same reaction but differ slightly in structure and kinetic properties
ex: lactate dehydrogenase - usually slightly different structure and different kinetics
- arise through genetics, duplications and mutations
reversible covalent modification
- catalytic enzymes are markedly altered by the (reversible) covalent attachment of a modifying group
- usually phorphoryl group
- modification by a phosphoryl group = phosphorylation
–> ATP is the phosphoryl donor of the reactions, catalyzed by protein kinases
–> removal of phosphoryl groups by hydrolysis is carried out by protein phosphatases
–> ex: glycogen phosphorylase
- phosphorylation cascade
proteolytic activation
- enzymes can be irreversibly converted from an inactive state into an active one by proteolytic cleavage
- wait to activate them until they are needed, otherwise they would act on the wrong things
–> activation occurs via hydrolysis of at least one peptide bond in inactive precursors called zymoens or proenzymes
–> regulatory mechanism generates many active digestive and (blood) clotting enzynes
–> ex: chymotripsinogen/chymotrypsin
Controlling the amount of enzyme rpesent
enzyme activity can be regulated by adjusting the amount of enzyme present
- enhanced/upregulated or diminished/downgraded by a cell at the transcriptional, posttranscriptional or translational level in response to a change in cellular environment
- depend on the rate of enzyme degradation - post translational regulation strategy
- ex: ubiquitin proteasome pathway
labeled and recognized by a protein “shredder” because:
- dont need it anymore
- recycle ieces
Ex: allosteric enzyme aspartate transcarbamyolase
- ATCase catalyzes the first step in biosynthesis of pyrimidines
- condensation of aspartate and carbamoyl phosphate to form N-carbamoylaspartate and orthophosphare
- committed step in the metabolic pathway that will ultimately yield pyrimidine mucleotides such as cytidine triphosphaste (CTP)
How is ATCase inhibited by CTP?
- ATCase is inhibited by CTP, the final product of the pyrimidine synthesis pathway (feedback inhibition)
- CTP is structurally quite different from the substrates of the reaction
- it binds the allosteric or regulatory sites and acts as an allosteric inhibitor
(as CTP is increase, rate of N-carbamoylaspartate decreases)
Feedback inhibition
- final product of a metabolic pathway shuts down the pathway
- prevents a cell from wasting chemical resources by synthesizing more product than is needed
ATCase structure
- dodecamer with 12 subunits
- 2 catalytic trimers
- 3 regulatory dimers
(C3)2(r2)3
Interaction of PALA with ATCase
- PALA is a competitive inhibitor that binds to the active site
- it can only bind to it when it is in the r/reactive state
Conformations of ATCase
- compact, relatively inactive form called the tense (T or low substrate affinity state)
- expanded form called Relaxed (R or high subs affinity) state
T to R trantision
CTP holds ATCase in the T state
- it moves into that conformation itself, and CTP keeps it there
Kinetics and ATCase
- do not display michaelis-menten kinetics
–> binding of substrate to one active site of the enzyme increases the activity at the other active sites (cooperativity)
- not a steady increase, instead is an on/off switch
Homotropic and heterotropic regulation
Homotropic = 1 at a time
- concerted: all are in R state and bind substrate one at a time
- sequential: all begin in T state, convert to R one at a time and bind substrate one at a time
heterotropic = all at once
- either all in R with effector
- all in T with effector
*see diagram
Homotropic regulation
ATP and CTP effects on ATCase?
- ATP activates ATCase
- CTP inhibits
- both are allosteric EFFECTORS
ATP is a purine, in DNA synthesis equal amounts of purine and pyrimidine are needed, so if ATP is produced it will trigger pyrimidine production
Protein phosphorylation mechanism
Insulin release mechanism
