Protein Function: Myoglobin, Hemoglobin, Muscle, and Antibodies

Flashcards covering key concepts from Chapter 7 on Protein Function, including myoglobin, hemoglobin, muscle contraction, and antibodies.

What are some of the specialized biological tasks that proteins carry out?

Proteins serve as enzyme catalysts for metabolic reactions, and participate in building, supporting, recognizing, transporting, and transforming cellular components.

What is the primary difference in oxygen-binding behavior between myoglobin and hemoglobin?

Myoglobin is a small protein with relatively simple oxygen-binding behavior, while hemoglobin is a tetramer of myoglobin-like polypeptides that provides a sophisticated system for delivering oxygen to tissues.

What key prosthetic group is found in myoglobin and hemoglobin that is responsible for oxygen binding?

Both myoglobin and hemoglobin contain a single heme group.

In the heme group, which atom directly binds to oxygen and what is its oxidation state?

The Fe(II) atom at the center of the heme group directly binds to oxygen.

Why are molecules like CO, NO, and H2S highly toxic when inhaled?

These molecules can bind to heme groups in proteins with much higher affinity than O2, which accounts for their toxicity.

What is the major physiological role of myoglobin in muscle cells?

Myoglobin's major physiological role is to facilitate oxygen diffusion and increase the effective solubility of O2 in muscle cells.

How is the oxygen-binding capacity of myoglobin characterized?

It is characterized by its fractional saturation (YO2), which is the fraction of O2-binding sites occupied by O2.

What type of binding curve describes oxygen binding to myoglobin, and what does it imply about ligand interaction?

Oxygen binding to myoglobin is described by a hyperbolic binding curve, which implies that ligands interact independently with their binding sites.

What is the physiological significance of hemoglobin having a p50 of 26 torr, compared to myoglobin's p50 of 2.8 torr?

Hemoglobin's higher p50 (lower affinity) means it can deliver much more O2 to tissues because it is nearly fully saturated at arterial oxygen pressures but only half-saturated at venous oxygen pressures, allowing for efficient release.

What is the quaternary structure of hemoglobin?

Hemoglobin is an α2β2 tetramer, meaning it consists of two alpha and two beta subunits.

What type of curve describes oxygen binding to hemoglobin, and what does it indicate about its binding sites?

Oxygen binding to hemoglobin is described by a sigmoidal (S-shaped) curve, which is diagnostic of cooperative interactions between binding sites.

What is the Hill constant (n) and what does its value indicate about cooperativity?

The Hill constant (n) describes the degree of cooperativity of a reaction; n=1 indicates noncooperative binding, n>1 indicates positively cooperative binding, and n<1 indicates negatively cooperative binding.

What are the two stable conformational states of hemoglobin, and to which oxygenation state do they correspond?

The two stable conformational states are the T state (corresponding to deoxyhemoglobin) and the R state (corresponding to oxyhemoglobin).

How does oxygen binding initiate the shift from the T state to the R state in hemoglobin?

Oxygen binding changes the heme's electronic state, shortening the Fe—N porphyrin bonds, which drags the covalently linked His F8 and leads to a series of coordinated movements shifting the protein from the T state to the R state.

What is the Bohr effect in the context of hemoglobin's oxygen transport?

The Bohr effect describes how decreasing the pH (increasing proton concentration) decreases hemoglobin's oxygen affinity, leading to the release of oxygen in metabolically active tissues where CO2 production lowers pH, and conversely, increasing pH stimulates hemoglobin to bind more O2.

How does dissolved CO2 directly affect hemoglobin's oxygen binding affinity?

CO2 directly combines with the N-terminal amino groups of hemoglobin to form carbamates, which stabilize the T (deoxy) state, thus favoring the release of O2.

What is the role of D-2,3-bisphosphoglycerate (BPG) in regulating hemoglobin's oxygen affinity?

BPG binds tightly to deoxyhemoglobin (T state) but weakly to oxyhemoglobin (R state), thereby decreasing hemoglobin's oxygen affinity and stabilizing the deoxy conformation, facilitating oxygen release to tissues.

How does the binding of BPG affect fetal hemoglobin differently than adult hemoglobin?

BPG binds more tightly to adult hemoglobin than to fetal hemoglobin, which results in the higher oxygen affinity of fetal hemoglobin, facilitating the transfer of O2 from the mother to the fetus.

What are allosteric effects?

Allosteric effects are interactions where the binding of a ligand at one site affects the binding of another ligand at a different site, typically requiring interactions among subunits of oligomeric proteins.

What is sickle-cell anemia caused by?

Sickle-cell anemia is caused by a mutation in hemoglobin S where Valine replaces Glutamate at the sixth position of each β chain, leading to the formation of rigid fibers that deform erythrocytes.

What is the name of the model that describes how thick and thin filaments interact during muscle contraction?

The sliding filament model describes how interdigitated thick and thin filaments slide past each other during muscle contraction.

What are the major protein components of thick and thin filaments in muscle?

Thick filaments are primarily composed of myosin, and thin filaments are primarily composed of actin.

What is the role of the myosin head in muscle contraction?

The myosin head is an ATPase that forms cross-bridges with thin filaments and converts the chemical energy of ATP hydrolysis into the mechanical energy of movement.

What two key proteins, besides actin and myosin, are involved in regulating muscle contraction by controlling myosin head access to actin?

Tropomyosin and troponin are the two proteins that regulate muscle contraction by controlling the access of myosin heads to their binding sites on actin.

How does calcium (Ca2+) regulate muscle contraction?

A nerve impulse releases Ca2+ from the sarcoplasmic reticulum, which binds to troponin C and triggers a conformational change in the troponin–tropomyosin complex, increasing myosin's affinity for actin and initiating contraction.

What are antibodies (immunoglobulins) and which cells produce them?

Antibodies are proteins produced by B lymphocytes (B cells) that recognize and bind to foreign macromolecules called antigens as part of the immune response.

What is the basic structural composition of all immunoglobulins?

contain at least four subunits: two identical light chains (L) and two identical heavy chains (H), associating via disulfide bonds to form a Y-shaped molecule with the formula (LH)2.

Which region of an antibody is responsible for recognizing specific antigens?

The antigen recognition ability resides in three loops within the variable domain (VL and VH), located at the tip of each Fab fragment in a crevice.

What is unique about 'memory B cells' in the immune system?

Memory B cells are long-lived and can recognize an antigen encountered previously, allowing them to mount a more rapid and massive immune response upon subsequent exposure.

What causes an autoimmune disease?

An autoimmune disease occurs when the immune system loses its tolerance to some of its own self-antigens, mistakenly attacking the body's own tissues.