Biochem Exam 1: Set 5 (Reactive Oxygen Species)

Question 1

What are some beneficial uses of free radicals?

Answer 1

synthesis of steroid hormones
kill bacteria in phagocytes
synthesis of some nucleotides
act as a paracrine hormone

Question 2

What are some sources of uncontrolled (and harmful) free radicals?

Answer 2

ischemic reperfusion injury
organ transplant
chemotherapy
aging
neurodegenerative diseases

Question 3

What is ischemic reperfusion injury?

Answer 3

oxidative damage to tissue caused by free radicals when blood flow is restored

Question 4

Define a radical and a free radical?

Answer 4

radical- a molecule with an unpaired electron

free radical- a radical that can diffuse about the cell

Question 5

What ROS are present in the body?

Answer 5

superoxide
hydroxide radical
(hydrogen peroxide is not a radical but can easily generate them)

Question 6

What compounds can catalyze the transfer of a single electron to oxygen making a superoxide radical?

Answer 6

Fe2+ and Cu+

Question 7

Describe the Haber-Weiss reaction

Answer 7

superoxide + H2O2 + H+ –> O2 + H2O + hydroxide radical

Question 8

Describe the Fenton reaction

Answer 8

H2O2 –> OH- + hydroxide radical (catalyzed by Fe2+ –> Fe3+)

Question 9

Which classes of enzymes use oxygen directly?

Answer 9

oxidases- transfer electrons to oxygen forming water or hydrogen peroxide

oxygenases- incorporate one or both oxygen atoms into a substrate

Question 10

What is gout?

Answer 10

Build up of uric acid in joints

Question 11

How does xanthine oxidase contribute to myocardial reperfusion injury?

Answer 11

it oxidizes xanthine to uric acid and generates hydrogen peroxide.

Explanation: Ca2+ builds up in tissue during ischemia which induces the proteolysis of xanthine dehydrogenase to xanthine oxidase. Upon reperfusion xanthine oxidase activity increases generating H2O2.

Question 12

Hydrogen peroxide from xanthine oxidase is converted to hydroxide radicals through what reaction?

Answer 12

Fenton reaction

Question 13

What are the functions of cytochrome p450?

Answer 13

synthesize certain steroid hormones

degrades exogenic compounds (drugs, toxins, etc.)

Question 14

What does C-p450 use as an electron source to add oxygen to substrates?

Answer 14

NADPH

Question 15

How is nitric oxide produced?

Answer 15

produced by NO synthase using arginine, NADPH, and molecular oxygen to form citrulline

Question 16

What is the function of NO in the body?

Answer 16

neurotransmitter and local hormone

causes vasodialation by binding to guanylyl cyclase

used in macrophages as a bactericide

Question 17

What are the cofactors of NO synthase?

Answer 17

Fe-Heme, FAD, FMN, BH4

Question 18

Where are superoxide anions produced?

Answer 18

produced by electron transport

can’t diffuse far before reacting

Question 19

What ROS is produced during fat degradation?

Answer 19

organic peroxide radicals

Question 20

What is the most biologically potent radical and how is it produced?

Answer 20

hydroxyl radical

produced from hydrogen peroxide by the Fenton reaction in the presence of Fe2+ or Cu+

Question 21

What are the major ROS production sites in mitochondria?

Answer 21

Fe-S centers of complex 1
partially reduced ubiquinone
cyt-b in complex 3
cytochrome oxidase

Question 22

What percentage of oxygen used by the mitochondria will be converted to a ROS

Answer 22

1-2%

Question 23

What lipases are a major source of ROS?

Answer 23

arachidonic acid oxidation

Question 24

What oxidases are a major source of ROS/NROS?

Answer 24

nitric oxide synthase
xanthine oxidase
NADPH oxidase
monoamine oxidase
p450 enzymes

Question 25

What chemical is used to measure the amount of reactive oxygen species in the body?

Answer 25

Malondialdenhyde (degradation product of organic peroxide radicals)

Question 26

Why are polyunsaturated fatty acids susceptible to reactions with free radicals?

Answer 26

conjugated double bonds stabilizes the fatty acid radical

Question 27

How can free radicals cause damage to DNA?

Answer 27

hydroxide radical can react with guanidine producing 8-hydroxyl-guanidine which is then converted to thymidine, leading to a mutation

Question 28

How can free radicals effect proteins?

Answer 28

free radicals can attack cysteine and cause crosslinking nitrated tyrosine may be linked to inflammatory disease

Question 29

What oxidative enzymes are used in macrophages to phagocytize bacteria?

Answer 29

iNOS SOD myeloperoxidase (NADPH oxidase is used extracellularly)

Question 30

What factors contribute to an increase in reactive oxygen species?

Answer 30

inflammation, radiation, aging, smog, chemicals, drugs, reperfusion, high pO2

Question 31

What cellular defense mechanisms are used to combat ROS's?

Answer 31

compartmentalization free radical scavengers (vitamin C and E) DNA repair mechanisms protective enzymes

Question 32

How does glutathione help prevent oxidative damage?

Answer 32

converted from a reduced to an oxidized form by glutathione peroxidase and converts hydrogen peroxide to water

Question 33

Where is glutathione peroxidase found and what cofactor is required in some isoforms?

Answer 33

found in mitochondria and cytosol some require selenium

Question 34

Besides glutathione peroxidase, what other enzymes are involved in the glutathione oxidative defense mechanism?

Answer 34

glutathione reductase- converts oxidized glutathione back to reduced form by converting NADPH to NADP+ glucose-6-phosphate dehydrogenase- maintains NADPH levels so glutathione reductase can keep glutathione reduced (especially important in red blood cells)

Question 35

What 2 enzymes function to defend the cell from oxidative damage?

Answer 35

superoxide dismutase catalase

Question 36

Where is superoxide dismutase found and what activates it?

Answer 36

different isozymes in cytosol, mitochondria, and extracellular matrix induced by oxidative stress

Question 37

What reaction is catalyzed by superoxide dismutase?

Answer 37

2 superoxide radicals + 2 H+ --> H2O2 + O2

Question 38

What reaction is catalyzed by catalase?

Answer 38

2 H2O2 --> 2 H2O

Question 39

Where is catalase found?

Answer 39

in peroxisomes and cytoplasm of phagocytes

Question 40

What factor limits the kinetics of catalase?

Answer 40

limited by the diffusion rate of peroxide in water (very fast enzyme)

Question 41

What are the major biological antioxidants?

Answer 41

vitamin E (tocopherol) vitamin C (ascorbate) vitamin A flavonoids

Question 42

What tissue is protected from oxidative damage by vitamin E?

Answer 42

vitamin E is lipophilic, protects against lipid membrane peroxidation

Question 43

What Vitamin A precursors may also function beneficially as antioxidants?

Answer 43

beta-carotene

Question 44

What is the major source of flavonoids?

Answer 44

food (primarily vegetables, wine, and beer)

Question 45

Describe ischemic postconditioning

Answer 45

interruption of reperfusion with short cycles of ischemia

Question 46

What event during reperfusion causes the opening of the mitochondrial transition pore?

Answer 46

abrupt increase in Ca2+