Hemoglobin (Hb) and myoglobin (Mb) are the O2-carrying proteins in vertebrates. Hb, which is contained within red blood cells, serves as the O2 carrier in blood and also plays a vital role in the transport of CO2 and H+. Vertebrate Hb consists of four polypeptides (subunits) each with a heme group. The four chains are held together by noncovalent attractions. The affinity of Hb for O2 varies between species and within species depending on such factors as blood pH, stage of development, and body size. For example, small mammals give up O2 more readily than large mammals because small mammals have a higher metabolic rate and require more O2 per gram of tissue.The binding of O2 to Hb is also dependent on the cooperativity of the Hb subunits. That is, binding at one heme facilitates the binding of O2 at the other hemes within the Hb molecule by altering the conformation of the entire molecule. This conformational change makes subsequent binding of O2 more energetically favorable. Conversely, the unloading of O2 at one heme facilitates the unloading of O2 at the others by a similar mechanism.Figure 1 depicts the O2-dissociation curves of Hb (Curves A, B, and C) and myoglobin (Curve D), where saturation, Y, is the fractional occupancy of the O2-binding sites. The fraction of O2 that is transferred from Hb as the blood passes through the tissue capillaries is called the utilization coefficient. A normal value is approximately 0.25. Figure 1Myoglobin facilitates transport in muscle and serves as a reserve store of O2. Mb is a single polypeptide chain containing a heme group, with a molecular weight of 18 kd. As can be seen in Figure 1, Mb (Curve D) has a greater affinity for than Hb.If Curve B represents the O2-dissociation curve for elephant Hb, which curve most closely resembles the curve for mouse Hb?
Answer(s): C
According to the passage, small mammals have higher metabolic rates and require a greater amount of oxygen per gram of tissue than larger mammals, and as a result, have hemoglobin that dissociates oxygen more readily than the hemoglobin of large mammals. A high metabolic rate implies that there's a whole lot of aerobic respiration going on. Metabolically active tissue needs lots of oxygen. The benefit of having Hb that easily dissociates oxygen is that when hemoglobin delivers oxygen to metabolically active tissue, it will readily give up its oxygen to the tissue. This means that for a given value of oxygen pressure, mouse hemoglobin will be less saturated with oxygen than elephant hemoglobin, since an elephant is much larger than a mouse and therefore has a much lower metabolic rate. In terms of Figure 1, the mouse Hb curve will be to the right of the elephant Hb curve. So if Curve B represents oxygen dissociation for elephant hemoglobin, then Curve C most closely resembles the curve for mouse Hb. Therefore, choices A and B are wrong and choice C is correct. As for choice D, we're twice told in the passage that Curve D represents oxygen dissociation for myoglobin. So Curve D isn't even an option since we're dealing with hemoglobin in this question.
Hemoglobin (Hb) and myoglobin (Mb) are the O2-carrying proteins in vertebrates. Hb, which is contained within red blood cells, serves as the O2 carrier in blood and also plays a vital role in the transport of CO2 and H+. Vertebrate Hb consists of four polypeptides (subunits) each with a heme group. The four chains are held together by noncovalent attractions. The affinity of Hb for O2 varies between species and within species depending on such factors as blood pH, stage of development, and body size. For example, small mammals give up O2 more readily than large mammals because small mammals have a higher metabolic rate and require more O2 per gram of tissue.The binding of O2 to Hb is also dependent on the cooperativity of the Hb subunits. That is, binding at one heme facilitates the binding of O2 at the other hemes within the Hb molecule by altering the conformation of the entire molecule. This conformational change makes subsequent binding of O2 more energetically favorable. Conversely, the unloading of O2 at one heme facilitates the unloading of O2 at the others by a similar mechanism.Figure 1 depicts the O2-dissociation curves of Hb (Curves A, B, and C) and myoglobin (Curve D), where saturation, Y, is the fractional occupancy of the O2-binding sites. The fraction of O2 that is transferred from Hb as the blood passes through the tissue capillaries is called the utilization coefficient. A normal value is approximately 0.25.Figure 1Myoglobin facilitates transport in muscle and serves as a reserve store of O2. Mb is a single polypeptide chain containing a heme group, with a molecular weight of 18 kd. As can be seen in Figure 1, Mb (Curve D) has a greater affinity for than Hb.If Curve B represents the O2-dissociation curve for human adult Hb, which of the following best explains why Curve A most closely resembles the curve for fetal Hb?
Fetuses are 100% dependent on their mothers for all of their nutritional needs, oxygen being one of them.Oxygen is delivered to the fetus by way of diffusion across the placentA. According to the question stem, Curve A most closely resembles the oxygen-dissociation curve for fetal hemoglobin assuming that Curve B is the curve for adult hemoglobin. This means that a given oxygen pressure, fetal hemoglobin is more saturated with oxygen than adult hemoglobin is. This implies that fetal hemoglobin has a greater affinity for oxygen than adult hemoglobin. In fact, at low partial pressures of oxygen, fetal hemoglobin has a 20-30% greater affinity for oxygen than adult hemoglobin. That is why oxygen binds preferentially to fetal hemoglobin in the capillaries of the placentA. Thus, choice C is correct and choices A, B, and D are wrong. In addition, fetal blood has a 50% higher concentration of hemoglobin than maternal blood, which increases the amount of oxygen that enters fetal circulation.
Hemoglobin (Hb) and myoglobin (Mb) are the O2-carrying proteins in vertebrates. Hb, which is contained within red blood cells, serves as the O2 carrier in blood and also plays a vital role in the transport of CO2 and H+. Vertebrate Hb consists of four polypeptides (subunits) each with a heme group. The four chains are held together by noncovalent attractions. The affinity of Hb for O2 varies between species and within species depending on such factors as blood pH, stage of development, and body size. For example, small mammals give up O2 more readily than large mammals because small mammals have a higher metabolic rate and require more O2 per gram of tissue.The binding of O2 to Hb is also dependent on the cooperativity of the Hb subunits. That is, binding at one heme facilitates the binding of O2 at the other hemes within the Hb molecule by altering the conformation of the entire molecule. This conformational change makes subsequent binding of O2 more energetically favorable. Conversely, the unloading of O2 at one heme facilitates the unloading of O2 at the others by a similar mechanism.Figure 1 depicts the O2-dissociation curves of Hb (Curves A, B, and C) and myoglobin (Curve D), where saturation, Y, is the fractional occupancy of the O2-binding sites. The fraction of O2 that is transferred from Hb as the blood passes through the tissue capillaries is called the utilization coefficient. A normal value is approximately 0.25.Figure 1Myoglobin facilitates transport in muscle and serves as a reserve store of O2. Mb is a single polypeptide chain containing a heme group, with a molecular weight of 18 kd. As can be seen in Figure 1, Mb (Curve D) has a greater affinity for than Hb. The sigmoidal shape of the O2-dissociation curve of Hb is due to:
Answer(s): D
The sigmoidal shape of the oxygen-dissociation curve for hemoglobin can be explained by the cooperativity among the subunits of the hemoglobin molecule. According to the passage, hemoglobin is composed of four subunits, each with its own heme group. Each heme unit is capable of binding to one molecule of oxygen, and so the entire molecule is capable of binding four molecules of oxygen. The binding of oxygen at the first heme group induces a conformational change in the hemoglobin molecule such that the second heme group's affinity for oxygen increases. Likewise, the binding of oxygen at the second heme group increases the third heme's affinity for oxygen; and the binding of oxygen at the third heme groups increases the fourth's affinity for oxygen.Therefore, the partial pressure of oxygen and the % oxygen-saturation of hemoglobin are NOT linearly proportional. As a consequence of these shifts in oxygen affinity with each binding, the line representing the oxygen-dissociation curve for hemoglobin is not straight, but rather a sigmoidal, or S-shaped, curve. Thus, choice D is the right answer. Let's take a look at the wrong answers for a minute. When the iron molecule of the heme group binds to oxygen, it is reduced; when the iron releases the oxygen, it is oxidized. However, this neither results in the sigmoidal shape of the curve, nor does it affect it. So, choice A is wrong. The concentration of carbon dioxide in the blood, choice B, is a factor that does affect hemoglobin's affinity for oxygen, and therefore affects the positioning of the curve, but it is NOT RESPONSIBLE for the sigmoidal shape. A high concentration of carbon dioxide in the blood will decrease hemoglobin's affinity for oxygen, and will therefore shift the curve to the right. So, choice B is also wrong. Choice C is also a true statement myoglobin does have a higher affinity for oxygen than does hemoglobin, as shown in Figure 1. However, this does not affect the shape of the sigmoidal curve. Therefore, choice C is also wrong.
Hemoglobin (Hb) and myoglobin (Mb) are the O2-carrying proteins in vertebrates. Hb, which is contained within red blood cells, serves as the O2 carrier in blood and also plays a vital role in the transport of CO2 and H+. Vertebrate Hb consists of four polypeptides (subunits) each with a heme group. The four chains are held together by noncovalent attractions. The affinity of Hb for O2 varies between species and within species depending on such factors as blood pH, stage of development, and body size. For example, small mammals give up O2 more readily than large mammals because small mammals have a higher metabolic rate and require more O2 per gram of tissue.The binding of O2 to Hb is also dependent on the cooperativity of the Hb subunits. That is, binding at one heme facilitates the binding of O2 at the other hemes within the Hb molecule by altering the conformation of the entire molecule. This conformational change makes subsequent binding of O2 more energetically favorable. Conversely, the unloading of O2 at one heme facilitates the unloading of O2 at the others by a similar mechanism.Figure 1 depicts the O2-dissociation curves of Hb (Curves A, B, and C) and myoglobin (Curve D), where saturation, Y, is the fractional occupancy of the O2-binding sites. The fraction of O2 that is transferred from Hb as the blood passes through the tissue capillaries is called the utilization coefficient. A normal value is approximately 0.25. Figure 1Myoglobin facilitates transport in muscle and serves as a reserve store of O2. Mb is a single polypeptide chain containing a heme group, with a molecular weight of 18 kd. As can be seen in Figure 1, Mb (Curve D) has a greater affinity for than Hb.A sample of human adult Hb is placed in an 8 M urea solution, resulting in the disruption of noncovalent interactions. After this procedure, the chains of Hb are isolated. Which of the four curves most closely resembles the O2-dissociation curve for the isolated chains? [Note: Assume that Curve B represents the O2- dissociation curve for human adult Hb in vivo.]
From the passage you know that the four subunits in Hb are held together by noncovalent interactions. So placing a sample of human adult hemoglobin in an 8 M urea solution, which you're told disrupts noncovalent interactions, will cause the subunits to break apart. You're also told in the question stem that the alpha chains of this sample of hemoglobin were isolated. So you need to figure out what the oxygen dissociation curve of a single peptide chain would look like. From the passage you also know that myoglobin consists of a single polypeptide chain. Therefore, the oxygen-dissociation curve for one polypeptide chain of Hb would be expected to look similar to the curve for myoglobin. In fact, both the individual alpha chains and the beta chains of hemoglobin resemble the tertiary structure of myoglobin. Thus, the curve for the alpha chain will look like Curve D and so, choice D is correct. The single chain of Hb will NOT look like Curves A, B or C because these curves have a unique shape due to the cooperativity of the four hemoglobin subunits. And since you're now dealing with a single chain, because of that treatment with an 8 M urea solution, no cooperativity is possible.
Hemoglobin (Hb) and myoglobin (Mb) are the O2-carrying proteins in vertebrates. Hb, which is contained within red blood cells, serves as the O2 carrier in blood and also plays a vital role in the transport of CO2 and H+. Vertebrate Hb consists of four polypeptides (subunits) each with a heme group. The four chains are held together by noncovalent attractions. The affinity of Hb for O2 varies between species and within species depending on such factors as blood pH, stage of development, and body size. For example, small mammals give up O2 more readily than large mammals because small mammals have a higher metabolic rate and require more O2 per gram of tissue.The binding of O2 to Hb is also dependent on the cooperativity of the Hb subunits. That is, binding at one heme facilitates the binding of O2 at the other hemes within the Hb molecule by altering the conformation of the entire molecule. This conformational change makes subsequent binding of O2 more energetically favorable. Conversely, the unloading of O2 at one heme facilitates the unloading of O2 at the others by a similar mechanism.Figure 1 depicts the O2-dissociation curves of Hb (Curves A, B, and C) and myoglobin (Curve D), where saturation, Y, is the fractional occupancy of the O2-binding sites. The fraction of O2 that is transferred from Hb as the blood passes through the tissue capillaries is called the utilization coefficient. A normal value is approximately 0.25.Figure 1Myoglobin facilitates transport in muscle and serves as a reserve store of O2. Mb is a single polypeptide chain containing a heme group, with a molecular weight of 18 kd. As can be seen in Figure 1, Mb (Curve D) has a greater affinity for than Hb.The utilization coefficient is continually being adjusted in response to physiological changes. Which of the following values most likely represents the utilization coefficient for human adult Hb during strenuous exercise?
You're told that the utilization coefficient is the fraction of the blood that releases its oxygen to tissues under normal conditions, and that under these conditions, the value of the coefficient is approximately 0.25. During strenuous exercise, there is a greater demand for oxygen, especially in skeletal muscle, where oxygen supplies are rapidly depleted during cellular respiration. Under such conditions, one would expect that a greater fraction of the blood would give up its oxygen, and that the utilization coefficient would therefore be some value greater than 0.25. Since choice D is the only value greater than 0.25, choice D is the right answer, and choices A, B, and C are wrong. During strenuous exercise, the utilization coefficient has been recorded at values ranging between 0.75 and 0.85 meaning that 75-85% of the blood gives up its oxygen in tissue capillaries.
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