-loss of allosteric properties-ATCase can become desensitized by treatment with mercurial compounds that react w/ the sulfhydryl groups in cysteine
1. multiple active sites (quaternary structure)2. active sites can exist in T or R state3. all active sites must be in the same state4. the T state is more stable
enzymes that differ in a.a. sequence yet catalyze the same rxn-come about by gene duplication-display different KMs
The enzyme catalyzes the first step in the synthesis of pyrimidines. It facilitates the condensation of carbamoyl phosphate and aspartate to form N-carbamoylaspartate and inorganic phosphate.
You are watching: An allosteric enzyme that follows the concerted mechanism
The inhibition of an allosteric enzyme by the end product of the pathway controlled by the enzyme. It prevents production of too much end product and the consumption of substrates when product is not required.
All of the enzyme would be in the R form all of the time. There would be no cooperativity. The kinetics would look like that of a michaelis-menten enzyme.
An allosteric enzyme that follows the concerted mechanism has a T/R ratio of 300 in the absence of substrate. Suppose that a mutation reversed the ratio. How would this mutation affect the relation between the rate of the reaction and the substrate concentration?
The enzyme would show simple Michaelis-Menten kinetics because it is essentially always in the R state.
CTP inhibits ATCase; however, the inhibition is not complete. Can you suggest another molecule that might enhance the inhibition of ATCase?
Homotropic = substrates of allosteric enzymes, account for the sigmoidal nature of the velocity vs. substrate concentration curve.Heterotropic = regulators of allosteric enzymes, alter the midpoint of KM of the curve.Both effectors work by altering the R/T ratio.
If isolated regulatory subunits and catalytic subunits of ATCase are mixed, the native enzyme is reconstituted. What is the biological significance of this observation?
the complex quaternary structure and the resulting catalytic and regulatory properties are ultimately encoded in the primary structure of the individual components.
X-ray crystallographic studies of ATCase in the R form required the use of the bisubstrate analog PALA. Why was this analog, a competitive inhibitor, used instead of the actual substrates?
If substrates had been used the enzyme would catalyze the reaction. Intermediates would not accumulate on the enzyme. Consequently any enzymes that crystallized would have been free of substrates or products.
PALA is a potent inhibitor of ATCase because it mimics the two biological substrates. However, low concentrations of this unreactive bisubstrate analog increase the reaction velocity. On the addition of PALA, the reaction rate increases until an average of three molecules of PALA are bound per molecule of enzyme. This max velocity is 17-fold greater than it is in the absence of PALA. The rxn rate then decreases to nearly zero on addition of three of more molecules of PALA per molecule of enzume. Why do low
The binding of PALA switches ATCase from the T state to the R state b/c PALA is a substrate analog. An enzyme molecule containing bound PALA has fewer free catalytic sites than does an unoccupied enzyme molecule. However, the PALA-containing enzyme will be in the R state and hence have a higher affinity for the substrates. The dependence of the degree of activation on the concentration of PALA is a complex function of the allosteric function L0 and of the binding affinities of the R and T states for the analog and substrates.
The phosphorylation and dephosphorylation of proteins is a vital means of regulation. Protein kinases attach phosphoryl groups, whereas only a phosphatase will remove the phosphoryl group from the target protein. What is the energy cost of this means of covalent regulation?
Isozymes are homologous enzymes that catalyze the same rxn but have different kinetic or regulatory properties.
Hemoglobin A inhibits the formation of the long fibers of HbS and the subsequent sicking of the red cell on deoxygenation. Why does HbA have this effect?
Deoxy HbA contians a complementary site, and so it can add on to a fiber of deoxy HbS. The fiber cannot then grow further, because the terminal deoxy HbA molecule lacks a sticky patch.
2,3 BPG lies in a central cavity within the Hb tetramer, stabilizing the T state. What would be the effect of mutations that placed the BPG-binding site on the surface of the Hb?
The electrostatic interactions b/w BPG and Hb would be weakened by competition with water molecules. The T state would not be stabilized.
Glucose reacts slowly with Hb and other proteins to form covalent compounds. Why is glucose reactive? What is the nature of the adduct formed?
Glucose is reactive because of the presence of an aldehyde group in its open chain form. The aldehyde group slowly condenses with amino groups to form aldimine products of a type called Schiff-base adducts
Glucose and fructose are reducing sugars. Sucrose, or table sugar, is a disaccharide consisting of both fructose and glucose. Is sucrose a reducing sugar? Explain.
No. The anomeric carbon atom acts as the reducing agent in both glucose and fructose but, in sucrose, the anomeric carbon atoms of fructose an glucose are joined by a covalent bond and are thus not available to react.
Glycogen is a polymer of glucose linked by alpha 1-4 glycosidic bonds with branches formed approx. every 10 glucose units by alpha 1,6 glycosidic bonds. Starch consists of 2 polymers of glucose. Amylose is a straight-chain polymer formed by an alpha 1,4 glycosidic bonds. Amylopectin is similar to glycogen but has fewer branches, ~1 branch every 30 glucose units.
Cellulose is a linear polymer of glucose joined by beta 1,4 linkages.Glycogen is a branched polymer with the main chain being formed by alpha 1,4 glycosidic bonds. The beta linkages allow the formation of a linear polymer ideal for structural roles.The alpha 1,4 linkages of glycogen form a helical structure, which allows the storage of many glucose moieties in a small space.
the attachment of carbohydrate allows EPO to say in circulation longer and thus to function for longer of periods of time than a carbohydrate-free EPO
1. simple glycoproteins – secreted proteins; play a variety of roles; protein component constitutes the bulk of the mass2. proteoglycans – have glycosaminoglycans attached; play structural roles in cartilage and ECM; predominately carb3. mucins- lubricants, have multiple cars attached through an N-acetylgalactosamine moiety; predominately carb
The glycosaminoglycan, becasue it is heavily charged, binds many water molecules. When cartilage is stressed, such as when your heel hits the ground, the water is released, thus cushioning the impact. When you lift your heel, the water rebinds.
I-cell disease results when proteins normally destined to the lysosomes lack the appropriate carbohydrate-addressing molecule. Suggest another possible means by which I-cell disease might arise.
The lectin that binds mannose-6-phosphate might be defective and not recognize a correctly addressed protein.
Different molecular forms of a glycoprotein that differ in the amount of carbohydrate attached or the location of attachment or both.
The total collection of carbohydrates synthesized by a cell at particular times and under particular environmental conditions.
genome comprises all of the genes present in an organismproteome includes all possible protein products and modified proteins that a cell expresses under any particular set of circumstances. glycome consists of all the carbs synthesized by the cell under any particular set of circumstancesb/c genome is static but any given protein can be variously expressed and modified the proteome is more complex than the genome. the glycome, which includes not only the glycoforms of proteins, but also many possible car structure, must be even more complex
it suggests that carbs are on the cell surfaces of all organisms for the purpose of recognition by other organisms or the environment.
glycoprotein = a protein decorated w/ carbohydrateslectin = a protein that specifically recognizes carbohydrates. A lectin can also be a glycoprotein.
Why is it possible for an allosteric inhibitor to have no structural relation to the substrates or products of the reaction it inhibits?
Threshold effects – a small change in substrate concentration can cause a large change in reaction rate. there is more sensitivity to changes in substrate concentration.
Concerted: All molecules exist in the T state or in the R state. At each level of S binding, an equilibrium exists between T and R. The eq. shifts from strongly favoring the T state with no S bound to strongly favoring the R state when the molecule is fully loaded with substrate. The R state as a greater affinity for substrate than the T state.Sequential: The binding of a ligand changes the conformation of the subunit to which it binds. This conformational change induces changes in the neighboring subunits that increase their affinity for the ligand, but DO NOT CHANGE THEIR STATE
In its inactive state PKA is a tetramer consisting of 2 active units and 2 regulatory units. The regulatory units bind to and block the active sites of the active units with a pseudosubstrate sequence. When cAMP binds to the regulatory units, their structure is altered and the pseudosubstrate sequence dissociates from the active site of the active units.
Myoglobin – a single polypeptide chain formed of alpha helices connected by turns with 1 O-binding siteHemoglobin – 2 identical alpha beta dimers (a tetramer), has 4 O binding sites
In fetal RBCs the 2,3 BPG pocket isn”t formed as well, so the T-state isn”t stabilized as well. Thus, fetal Hb can bind oxygen under pO2s where mom gives up oxygen.
the increase in oxygen release promoted by H+ AND CO2. -Proton binding to certain residues in Hb facilitates the formation of ionic bonds that stabilize the T state.-CO2 reacts w/ n-terminal groups in Hb to form carbamate, which forms bonds w/in Hb to stabilize the T state.
a single a.a. substitution in the beta chain of hemoglobin is responsible (Val for Glu) creates a hydrophobic patch where there shouldn”t be one, allowing long HbS fibers to form that produce the characteristic sickle shape.
N-linked carbohydrates are synthesized in the _________ and modified in the ___________. O-linekd carbohydrates are attached in the ___________.
hemaglutinin (a viral surface protein) binds to specific carbohydrate residues present on cell surfaces. In humans, these receptors are only found very very deep in our lungs, so it is difficult for the virus to be breathed in deep enough to get there.