Protein Function

Myoglobin:

• Has quaternary structure.

• Contains a pocket for binding heme that is made up of

       largely the C and D helices

• Contains multiple binding sites for O2.

• Consists mostly of α helices

Answer explanation

Myoglobin is a single polypeptide of 153 amino acid residues with one molecule of heme. About 78% of the amino acid residues in the protein are found in the eight α helices typical of the globin fold, named A through H.

Oxygen carried by whole blood in animals is bound and transported by hemoglobin in what type of cell?

• Hemocytoblasts

• Erythrocytes

• White blood cells

• Stem cells

• Neurons

Answer explanation

Nearly all the oxygen carried by whole blood in animals is bound and transported by hemoglobin in erythrocytes (red blood cells). They are formed from precursor stem cells called hemocytoblasts.

Which molecule is a homotropic modulator of oxygen binding to hemoglobin?

• Oxygen

• H+

• Carbon dioxide

• Hemoglobin

• Carbon monoxide

Answer explanation

Cooperative binding of oxygen to hemoglobin, a multimeric protein, is a form of allosteric binding. The binding of one ligand affects the affinities of any remaining unfilled binding sites. Oxygen can be considered as both a ligand and an activating homotropic modulator.

In peripheral tissues:

• The affinity of hemoglobin for oxygen increases and the

       protein binds more O2

• CO2 is excreted

• The structural effects of H+ and CO2 binding to hemoglobin

      favor the R state

• O2 is released

Answer explanation

At the relatively low pH and high CO2 concentration of peripheral tissues, the affinity of hemoglobin for oxygen decreases as H+ and CO2 are bound, and O2 is released to the tissues.

Which statement is false?

• T lymphocytes produce immunoglobulins

• Immunoglobulins target foreign bacteria and viruses for

       destruction

• Memory cells allow the body to quickly respond to viruses

       it has encountered before

• Clonal selection increases the number of immune system

       cells that can respond to a particular pathogen

• Vaccines stimulate the production of memory cells

Answer explanation

T lymphocytes produce T-cell receptors on the surface of T cells. Recognition of infected cells or parasites involves these T-cell receptor proteins.