The process of depolarization triggers the cardiac cycle. The electronics of the cycle can be monitored by an electrocardiogram (EKG). The cycle is divided into two major phases, both named for events in the ventricle:the period of ventricular contraction and blood ejection, systole, followed by the period of ventricular relaxation and blood filling, diastole.During the very first part of systole, the ventricles are contracting but all valves in the heart are closed thus no blood can be ejected. Once the rising pressure in the ventricles becomes great enough to open the aortic and pulmonary valves, the ventricular ejection or systole occurs. Blood is forced into the aorta and pulmonary trunk as the contracting ventricular muscle fibers shorten. The volume of blood ejected from a ventricle during systole is termed stroke volume.During the very first part of diastole, the ventricles begin to relax, and the aortic and pulmonary valves close. No blood is entering or leaving the ventricles since once again all the valves are closed. Once ventricular pressure falls below atrial pressure, the atrioventricular (AV) valves open. Atrial contraction occurs towards the end of diastole, after most of the ventricular filling has taken place. The ventricle receives blood throughout most of diastole, not just when the atrium contracts.: Electronic and pressure changes in the heart and aorta during the cardiac cycle.Figure 1 According to Fig. 1, the opening of the aortic and pulmonary valves is NOT associated with:
Answer(s): C
Close analysis of Figure 1 (compare to previous questions especially question 2 reveals that during the period that the aortic and pulmonary valves are OPEN, the curve for left ventricular volume drops (= decreases) but does not rise (= increase).
Several models have been developed for relating changes in dissociation constants to changes in the tertiary and quaternary structures of oligomeric proteins. One model suggests that the protein's subunits can exist in either of two distinct conformations, R and T. At equilibrium, there are few R conformation molecules: 10 000 T to 1 R and it is an important feature of the enzyme that this ratio does not change. The substrate is assumed to bind more tightly to the R form than to the T form, which means that binding of the substrate favors the transition from the T conformation to R.The conformational transitions of the individual subunits are assumed to be tightly linked, so that if one subunit flips from T to R the others must do the same. The binding of the first molecule of substrate thus promotes the binding of the second and if substrate is added continuously, all of the enzyme will be in the R form and act on the substrate. Because the concerted transition of all of the subunits from T to R or back, preserves the overall symmetry of the protein, this model is called the symmetry model. The model further predicts that allosteric activating enzymes make the R conformation even more reactive with the substrate while allosteric inhibitors react with the T conformation so that most of the enzyme is held back in the T shape.Experiment Evaluating Non-Symmetry Model EnzymesExperiments were performed with enzyme conformers that did not obey the symmetry model. The data is summarized in Figure 1.Equilibrium distribution of two conformers at different temperatures given the free energy of their Figure 1:interconversion. (modified from Mr.Holmium).What assumption is made about the T and R conformations and the substrate?
Answer(s): A
Paragraph 1. Information concerning the relative amounts of T and R conformations present before substrate is added is given in the passage.
Several models have been developed for relating changes in dissociation constants to changes in the tertiary and quaternary structures of oligomeric proteins. One model suggests that the protein's subunits can exist in either of two distinct conformations, R and T. At equilibrium, there are few R conformation molecules: 10 000 T to 1 R and it is an important feature of the enzyme that this ratio does not change. The substrate is assumed to bind more tightly to the R form than to the T form, which means that binding of the substrate favors the transition from the T conformation to R.The conformational transitions of the individual subunits are assumed to be tightly linked, so that if one subunit flips from T to R the others must do the same. The binding of the first molecule of substrate thus promotes the binding of the second and if substrate is added continuously, all of the enzyme will be in the R form and act on the substrate. Because the concerted transition of all of the subunits from T to R or back, preserves the overall symmetry of the protein, this model is called the symmetry model. The model further predicts that allosteric activating enzymes make the R conformation even more reactive with the substrate while allosteric inhibitors react with the T conformation so that most of the enzyme is held back in the T shape.Experiment Evaluating Non-Symmetry Model EnzymesExperiments were performed with enzyme conformers that did not obey the symmetry model. The data is summarized in Figure 1.Equilibrium distribution of two conformers at different temperatures given the free energy of their Figure 1:interconversion. (modified from Mr.Holmium).The substrate binds more tightly to R because:
Answer(s): B
If a molecule has a high affinity for something, it is likely to be associated with it maximally. The substrate binds more tightly to the R conformation even though the R conformation is present in small amounts because R has a higher affinity for the substrate than T.
Several models have been developed for relating changes in dissociation constants to changes in the tertiary and quaternary structures of oligomeric proteins. One model suggests that the protein's subunits can exist in either of two distinct conformations, R and T. At equilibrium, there are few R conformation molecules: 10 000 T to 1 R and it is an important feature of the enzyme that this ratio does not change. The substrate is assumed to bind more tightly to the R form than to the T form, which means that binding of the substrate favors the transition from the T conformation to R.The conformational transitions of the individual subunits are assumed to be tightly linked, so that if one subunit flips from T to R the others must do the same. The binding of the first molecule of substrate thus promotes the binding of the second and if substrate is added continuously, all of the enzyme will be in the R form and act on the substrate. Because the concerted transition of all of the subunits from T to R or back, preserves the overall symmetry of the protein, this model is called the symmetry model. The model further predicts that allosteric activating enzymes make the R conformation even more reactive with the substrate while allosteric inhibitors react with the T conformation so that most of the enzyme is held back in the T shape.Experiment Evaluating Non-Symmetry Model EnzymesExperiments were performed with enzyme conformers that did not obey the symmetry model. The data is summarized in Figure 1.Equilibrium distribution of two conformers at different temperatures given the free energy of their Figure 1:interconversion. (modified from Mr.Holmium).The symmetry model describes a form of cooperative binding. Most enzymes do not engage in cooperative binding. The predicted shape of a graph representing reaction rate versus the addition of substrate to most enzymes would be expected to be:
The amount of substrate-enzyme complex would increase steadily as more substrate is added until a point at which all enzymes are involved in a substrate-enzyme complex, and any more substrate added will have no effect (saturation kinetics). The graph would show a steadily slowing curve of positive slope which reaches a point at which it levels off into a horizontal line. This curve is called a hyperbola (see image below). A sigmoidal shape would be expected in cooperative binding (i.e. the symmetry model as described in the passage or hemoglobin).Note: This was a classic question in the old MCAT and, not surprisingly, the same concept comes up in the AAMC's new MCAT practice materials: the difference between the simple (rectangular) hyperbola and the sigmoidal curve suggesting cooperative binding (and also, the ability to recognize the shapes of these 2 curves independently). Also note that the myoglobin saturation curve is a hyperbola, but hemoglobin has a sigmoid shape due to the cooperative binding of oxygen molecules.And finally, note the positions of Vmax (= maximum velocity/reaction rate) and Km (substrate concentration at 1/2 Vmax) displaying Michaelis-Menten kinetics associated with the hyperbolic curve on the left, as opposed to the sigmoidal curve on the right (image from the GS BIO book or ebook, BCM 2.9):
Several models have been developed for relating changes in dissociation constants to changes in the tertiary and quaternary structures of oligomeric proteins. One model suggests that the protein's subunits can exist in either of two distinct conformations, R and T. At equilibrium, there are few R conformation molecules: 10 000 T to 1 R and it is an important feature of the enzyme that this ratio does not change. The substrate is assumed to bind more tightly to the R form than to the T form, which means that binding of the substrate favors the transition from the T conformation to R.The conformational transitions of the individual subunits are assumed to be tightly linked, so that if one subunit flips from T to R the others must do the same. The binding of the first molecule of substrate thus promotes the binding of the second and if substrate is added continuously, all of the enzyme will be in the R form and act on the substrate. Because the concerted transition of all of the subunits from T to R or back, preserves the overall symmetry of the protein, this model is called the symmetry model. The model further predicts that allosteric activating enzymes make the R conformation even more reactive with the substrate while allosteric inhibitors react with the T conformation so that most of the enzyme is held back in the T shape.Experiment Evaluating Non-Symmetry Model EnzymesExperiments were performed with enzyme conformers that did not obey the symmetry model. The data is summarized in Figure 1.Equilibrium distribution of two conformers at different temperatures given the free energy of their Figure 1:interconversion. (modified from Mr.Holmium).The symmetry model would NOT account for an enzyme:
The symmetry model describes an instance of something which may be described as positive cooperativity (paragraph 2). The model does not exclude the enzyme from having cofactors, and places no restriction on what the enzyme's function will be. However, the symmetry model does not account for the existence of any other conformations than the two described (paragraph 2).
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