User:Eric Martz/Hemoglobin Quiz

Hemoglobin QuizHemoglobin Quiz

You will get immediate feedback when you click Submit. The quiz below is offered to accompany the interactive tutorial Hemoglobin Molecular Structure.

1

The number of protein chains in each hemoglobin molecule is

2

The number of heme-iron complexes in each hemoglobin molecule is

3

When saturated, the number of oxygen atoms bound by one molecule of hemoglobin is

Questions below may have more than one correct answer.

4 The function of hemoglobin is

	To pick up oxygen in the lungs.
	To convert oxygen to carbon dioxide and water.
	To transport oxygen.
	To release oxygen in the lungs.
	To release oxygen in body tissues.

5 The polypeptide chains that fit together to make one hemoglobin molecule are held together by

	disulfide bonds
	covalent bonds
	non-covalent interactions
	oxygen

6 The iron atom is held in place by direct contact with

	carboxyl groups in heme
	nitrogens in heme
	carbons in heme
	sulfur atoms in cysteines
	nitrogen atoms in histidines
	oxygen atoms in histidines

7 Molecular oxygen binds directly to, and is released from

	alpha helices
	oxygen atoms
	histidine amino acids
	cysteine amino acids
	iron atoms
	magnesium atoms

8 Heme groups are held

	in alpha chains
	in beta chains
	between alpha and beta chains
	in gamma chains

9 More than half of the atoms in heme are

	carbon
	oxygen
	nitrogen
	sulfur

10 Hydrogen bonds

	are covalent bonds
	are non-covalent bonds
	form between two hydrogen atoms
	stabilize the main chain conformation in alpha helices

11 Each chain in the hemoglobin molecule contains

	multiple beta strands
	multiple alpha helices
	a hydrophilic core
	a hydrophobic core
	one pocket to hold one heme
	one pocket to hold two hemes

12 An amphipathic alpha helix is one that

	occurs only in sickle disease
	occurs only in the de-oxy form of hemoglobin
	is hydrophilic on both sides
	has a hydrophilic side and a hydrophobic side
	is hydrophobic on both sides

13 Amino acids with hydrophobic sidechains are present mostly

	where they do not contact water
	on the surface of the protein molecule
	in order to bind molecular oxygen
	buried in the center of the protein molecule
	in separate helices from the helices of hydrophilic amino acids

14 Salt bridges

	form when oppositely-charged amino acid sidechains contact each other
	form when positively charged amino acid sidechains contact each other
	form mostly in the core of the molecule
	form mostly on the surface of the molecule
	are covalent bonds

15 Histidine amino acids

	have oxygen in their sidechains
	have nitrogen in their sidechains
	have carbon in their sidechains
	bind tightly to Fe++
	help to bind molecular oxygen

16 Oxygen binding

	is partly to a histidine sidechain nitrogen
	moves the iron in heme
	changes the conformation of hemoglobin
	is stronger at the higher pH in the lungs
	causes some salt bridges to separate
	is decreased by diphosphoglyceric acid

17 Sickle hemoglobin

	is advantageous in some parts of the world
	shortens the lifetime of red blood cells
	impairs blood flow
	is caused by a mutation that changes a single amino acid
	causes severe disease only in homozygotes
	polymerizes into fibers due to hydrophobic interactions

Suggestions to emartz at microbio dot umass dot edu.

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Answers to Questions in the TutorialAnswers to Questions in the Tutorial

Here are answers to the questions posed in green within the Hemoglobin Molecular Structure Tutorial.

1. The main function of hemoglobin is to transport oxygen from the lungs to tissues throughout the body.
2. Each hemoglobin molecule is made up of four protein chains.
3. Each chain in the hemoglobin molecule has a pocket to hold one heme-iron complex. Thus, there are four heme-iron complexes in each hemoglobin molecule.
4. Each of the heme-iron complexes can bind one O₂ molecule. Thus, each hemoglobin molecule can bind up to eight atoms of oxygen.
5. The four chains that make up one hemoglobin molecule are held together by non-covalent interactions (non-covalent bonds).
6. Each iron atom is held by five nitrogen atoms: four nitrogens in heme, plus one nitrogen in the sidechain of a histidine amino acid.
7. Molecular oxygen binds between iron (Fe⁺⁺) and a nitrogen in the sidechain of histidine. The oxygen-binding histidine is on the opposide side of the iron from the iron-binding histidine.
8. Alpha helices and loops (including turns) make up the alpha and beta chains of hemoglobin. They contain no beta strands or sheets.
9. Hydrogen bonds stabilize the conformation of an alpha helix.
10. Hydrogen bonds are non-covalent bonds.
11. Amphipathic means having one part that is hydrophobic, and another part that is hydrophilic.
12. Hydrophobic (literally "water fearing") means structures that avoid contact with water because such contact is energetically unfavorable. Hydrophobic structures are apolar. In contrast, hydrophilic structures favor contact with water molecules, usually because they can form hydrogen bonds with water. Hydrophilic structures include polar and charged structures.
13. The protein pocket that holds heme is predominantly hydrophobic, as is heme itself. Thus, the avoidance of contact with water is partly what holds the heme in the pocket. The histidine that contacts the iron in heme is an exception, since its sidechain is hydrophilic.
14. Protein chains or domains that are soluble in water, such as the alpha and beta chains of hemoglobin, have hydrophobic cores. Amino acids with hydrophilic sidechains are mostly on the surfaces of these chains.
15. The hydrophobic cores of proteins are devoid of water. It is the avoidance of water that drives amino acids with hydrophobic sidechains to pack together in the core, while the attraction of hydrophilic amino acids to water favors their locations on the surface of the folded protein chain.
16. A slightly acidic pH, around 6.5, increases the partial charges on the sidechains of histidines. This is because their sidechain [pKa's are about 6.5. In contrast, lysine and arginine have full positive charges at any physiological pH because their sidechain pKa's are 11-12, while aspartic acid and glutamic acid have full negative charges at any physiological pH because their sidechain pKa's are about 4.
17. A salt bridge is a pair of opposite charges held together by electrostatic attraction. Oppositely charged amino acid sidechains are generally salt-bridged when their charged atoms are less than 4 Å apart.
18. One histidine (sequence number 87 in the beta chain) binds to iron (Fe⁺⁺), helping to hold the iron and heme in place. The other histidine (sequence number 58 in the beta chain) helps to bind molecular oxygen, and probably also serves as a gate to admit and release oxygen (see text under View 11 in Chapter 2).
19. Nitrogen. The lone-electron pairs of nitrogen are attracted to electron-seeking Fe⁺⁺ and O².
20. When oxygen binds to the Fe⁺⁺ in heme, the Fe⁺⁺ moves into the plane of the heme. This pulls the histidine attached to the iron, which in turn pulls the alpha helix containing that heme, which in turn favors the conformational change of the entire hemoglobin molecule from the low-oxygen-affinity deoxy T state to the higher oxygen-affinity oxy R conformation.