Flavin coenzymes

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Transcript Flavin coenzymes

Chapter 8. Vitamins and Enzyme Cofactors
I. Introduction
II. Fat-soluble vitamins (脂溶性维生素)
III. Water-soluble vitamins and coenzymes
(水溶性维生素和辅酶)
IV. Metal ions as cofactors (作为辅酶的金
属离子)
1
I. Introduction

Vitamins are compounds that are essential to the
health of humans and other vertebrates but
cannot be synthesized by these animals and must
therefore be obtained in the diet (生物体发育所
需但自身不能合成,需从食物中摄取).
 Requirements (需要量): g-mg per day
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Vitamin deficiencies and sources
Disease
Deficient
vitamins
Vitamin sources
Night blindness (夜盲症)
A
Liver oils, liver, eggs, whole milk,
butter, veggies
Beriberi (脚气病)
B1
Seeds, nuts, wheat germs, legumes,
lean meat
Pellagra (糙皮病)
B3
Meats, nuts, legumes
Pernicious anemia (恶性贫血)
B12
Liver, kidney, egg, cheese
Scurvy (坏血病)
C
Citrus and soft fruits
Rickets (软骨病, 佝偻病)
D
Milk, fortified food, fish oils, egg
yolks, liver
Hemolytic anemia (溶血性贫血) E
Veggie oil, nuts
Bleeding disorders
Green leafy veggies, fruits, dairy
products, veggie oils, cereals, meats
K
3
History
1919 – Vitamin A
1922 – Vitamin D
1923 – Vitamin E
1926 – Thiamin (硫胺素,维生素B1)
1926 – Riboflavin (核黄素,维生素B2)
1928 – Bacterial synthesis of B vitamins
1928 – Vitamin C
1929 – Vitamin K
1930 – Pantothenic Acid (泛酸,维生素B5)
1932 – Choline
1934 – Vitamin B6
1935 – Niacin (烟酸,维生素B3)
1936 – Biotin (生物素,维生素H)
1943 – Folacin (叶酸)
1948 – Vitamin B12
4
Nobel Laureates and Their Work with Vitamins
Nobel Prize in Physiology or Medicine
Discovery of Vitamins
Christiaan Eijkman (1929)
Vitamin B1
Sir Frederick Gowland Hopkins (1929) Growth stimulating vitamins
Whipple, Minot, Murphy (1934)
Vitamin B12
Henrik Carl Peter Dam (1943)
Vitamin K
Isolation of Vitamins
Adolf Otto Reinhold Windaus (1928)
Albert Nagyrapolt (1937)
Vitamin D
Vitamin C
Edward Adelbert Doisy (1943)
Vitamin K
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Nobel Prize in Chemistry
Synthesis of Vitamins
Walter Norman Haworth (1937)
Paul Karrer (1937)
Robert Burns Woodward (1965)
Vitamin C
Vitamin A, B2
Vitamin B12
Structure of Vitamins
Paul Karrer (1937)
Richard Kuhn (1938)
Lord (Alexander R.) Todd (1957)
Vitamin A, B
Vitamin B2
Vitamin B12
Dorothy Crowfoot Hodgkin (1964) Vitamin B12
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Classification and functions of vitamins
Classification
Fat-soluble
(脂溶性)
Vitamins
A
D
E
K
Functions
Vision (视力)
Ca & P metabolism
Antioxidant (抗氧化剂)
Blood clotting (血凝)
Water-soluble B, C, H, PP, lipoic acid,
Components or
folic acid, etc.
precursors of coenzymes
(水溶性)
(except vitamin C)
Vitamin B1 – Thiamin (硫氨素)
Vitamin B2 – Riboflavin (核黄素)
Vitamin B3 – Niacin (烟酸)
Vitamin B5 – Pantothenic acid (泛酸)
Vitamin B6 – Pyridoxine (吡哆醛)
Vitamin B9 – Folic acid (叶酸)
Vitamin B12 – Cobalamin (钴胺素)
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II. Fat-soluble vitamins
1.
2.
3.
4.
Vitamin A
Vitamin D
Vitamin E
Vitamin K
Hormone precursors
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1. Vitamin A
P.434
 Uptake of Vitamin A: can be absorbed in the diet
II. Fat-soluble vitamins




from animal sources or synthesized from -carotene
from plant sources.
Structure of vitamin A (维生素A的结构)
Precursor and derivatives of vitamin A
(维生素A的前体和衍生物)
Basis of visual transduction (视觉转换)
Vitamin A deficiency (维生素A缺乏)
 Dryness of skin, eyes, mucous membranes
 Retarded development and growth
 Night blindness (夜盲症), the early symptom
used in diagnosing vitamin A deficiency.
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Structure of vitamin A:
isoprene derivatives
2
II. Fat-soluble vitamins
3
1
4
CH2OH
6
Vitamin A1
5
Present in livers of mammals and sea fish
CH2OH
Vitamin A2
Present in livers of freshwater fish
Both types of vitamin have the similar physiological functions,
but A2 shows only half of the activity as compared to A1.
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Vitamin A: Precursor and derivatives
-Carotene
(-胡萝卜素)
Retinol
(视黄醇)
CH2OH
Vitamin A1
[O]
Retinal
(视黄醛)
[O]
Retinoic acid
(视黄酸)
CHO
Opsin
Rhodopsin
(视蛋白) (视紫质)
A pigment to initiate the response of retina cells to light
COOH
Signal to epithelial cells
(上皮组织)
A hormone that regulates gene expression in skin
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2. Vitamin D
P.437
Two members of the vitamin D family:
Vitamin D
D3
D2
7-脱氢胆甾醇
Cholecalciferol Ergocalciferol
(胆钙化甾醇) (麦角钙化甾醇)
Produced in
Animal skin
Plant
Precursor
(前体)
7-Dehydrocholesterol
Ergosterol
(麦角固醇)
Prohormone
19 20
Vitamin D3
D3: Prohormone (Hormone precursor)
-- Biologically inactive
Hormone
A hormone regulating Ca & P homeostasis
-- Active form of vitamin D
1,25-二羟维生素D3
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Vitamin D deficiency  rickets/osteomalacia (佝偻病/骨软化)
Excess intake of vitamin D  hypervitaminosis (维生素过多症)
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P.438
3. Vitamin E
-Tocopherol
The most active form
 Vitamin E = a group of 8 tocopherols (生育酚)
 Vitamin E is a most important natural antioxidant (天然抗氧化剂)
-- Vitamin E provides the 1st line of defence against peroxidation (过氧
化) of polyunsaturated fatty acids contained in cellular membranes.
 Vitamin E and selenium act synergistically (与硒协同抗氧化).
-- GSH peroxidase (谷胱苷肽过氧化物酶) provides the 2nd line of
defence against peroxidation.
 Deficiency of vitamin E may give rise to anemia (贫血) of the newborn.
 Vitamin E may help prevent or delay coronary heart disease.
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4. Vitamin K
II. Fat-soluble vitamins



Vitamin K is a blood-clotting cofactor (凝血维生素)
Vitamin K is required for biosynthesis of blood clotting
factors.
Hemorrhagic disease of the newborn (新生儿的出血病)
is caused by vitamin K deficiency.
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III. Water-soluble vitamins
1. Vitamin B1: Thiamine and thiamine pyrophosphate
2. Vitamins related to adenine nucleotides
1) Nicotinic acid and the nicotinamide coenzymes
(NAD+/NADH, NADP+/NADPH)
2) Riboflavin (Vitamin B2) and the flavin coenzymes
(FAD/FADH2, FMN/FMNH2)
3) Pantothenic acid and coenzyme A
3. Vitamin B6 and pyridoxal phosphate
4. Vitamin B12: Cyanocobalamin
5. Vitamin C: Ascorbic acid
6. Biotin
7. Lipoic acid
8. Folic acid
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Typical water-soluble vitamins and coenzymes
Vitamin
Coenzyme
Entity transferred
Enzyme example
Vitamin B1
TPP
Aldehydes
Pyruvate decarboxylase
Vitamin B2
FAD, FMN
H
Succinate dehydrogenase
Vitamin B6
Pyridoxal
phosphate
Amino groups
(氨基)
Aspartate
aminotransferase
Vitamin B12
5’-Deoxyadenosylcobalamin
H, alkyl groups
(烷基)
Methylmalonyl-CoA
mutase
Biocytin
(生物胞素)
CO2
Propionyl-CoA
carboxylase
Vitamin C (抗坏血酸)
Biotin (生物素)
Pantothenic acid (泛酸) CoA (辅酶A)
Acyl groups (酰基) Acetyl-CoA carboxylase
Vitamin PP
NAD+, NADP+
H-
Alcohol dehydrogenase
Lipoic acid (硫辛酸)
Lipoamide
Acyl groups
Pyruvate dehydrogenase
Folic acid (叶酸)
Tetrahydrofolate One-carbon groups Thymidylate synthase
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Vitamins related to adenine nucleotides
(与腺嘌呤核苷酸有关的维生素)
Cofactors
Nicotinamide
coenzyme
(烟酰胺辅酶)
NAD+/NADH
NADP+/NADPH
Flavin coenzyme FAD/FADH/FADH2
FMN/FMNH/FMNH2
(黄素辅酶)
Coenzyme A
(辅酶A)
Binding to
enzyme
Transfer
Vitamin PP
(Vitamin B3)
Loose
:H(e-)
Vitamin B2
Tight
H
Vitamin source
CoA/Acyl-CoA
Pantothenic acid
(Vitamin B5)
Acyl
group
18
黄
素
腺
嘌
呤
二
核
苷
酸
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III. Water-soluble vitamins
烟酰胺腺嘌呤二核苷酸
P.443
1). Vitamin PP and Nicotinamide coenzymes
(维生素PP和烟酰胺辅酶)
Vitamin PP
NAD(P)+
Nicotinamide
Adenine
Dinucleotide
Nicotinic acid
(烟酸)
Trp
Nicotinamide
(烟酰胺)
(Vitamin B3)
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NAD+ – Nicotinamide adenine dinucleotide – coenzyme I
– 辅酶I (烟酰胺腺嘌呤二核苷酸)
NADP+ – Nicotinamide adenine dinucleotide phosphate – coenzyme II
– 辅酶II (烟酰胺腺嘌呤二核苷酸磷酸)
III. Water-soluble vitamins

Coenzymes to many dehydrogenases catalyzing
oxidoreduction reactions (作为脱氢酶的辅酶催
化氧化还原反应)
 NAD – oxidative pathways (e.g. citric acid cycle)
 NADP – Reductive pathways (e.g. pentose phosphate
pathway)

Key components of many metabolic pathways (是
许多代谢途径的关键组分)
e.g. carbohydrate, lipid amino acid metabolism

Lack of niacin causes the deficiency syndrome
pellagra (糙皮病, 即烟酸缺乏症)
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NAD+
(Oxidized form)
NADH + H+
(Reduced form)
O
OH
III. Water-soluble vitamins
H3C
C
H
Lactate dehydrogenase
(乳酸脱氢酶)
AH2 + NAD(P)+
Reduced
COOH
Pyruvate
(丙酮酸)
Lactate
(乳酸)
Oxidized
(氧化态)
C
H3C
COOH
Reduced
(还原态)
A + NAD(P)H + H+
Oxidized
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Nicotinamide coenzymes as electron carriers
(烟酰胺辅酶充当电子载体)
III. Water-soluble vitamins
Oxidized form
(氧化型)
NAD+
- + 2H+
+
2e
NADP+
Reduced form
(还原型)
Oxidative (氧化途径)
Catabolic (分解途径)
Reductive ( 还原途径)
Biosynthetic (合成途径)
H+ + NADH
NADPH
Electron acceptor
(电子受体)
Electron donor
(电子供体)
2 H = 2 H+ + 2 e = H: + H+
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Pro-R
Pro-S
2 H = 2 H+ + 2 e = H: + H+
The “+” sign in NAD(P)+  the charge on the N atom
The “H” in NAD(P)H  the H: that is added
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Loose association between a dehydrogenase
and its nicotinamide coenzymes
(脱氢酶和烟酰胺辅酶之间的松散结合)
Example: in the production of alcohol during fermentation of glucose by yeast cells,
A. Glyceraldehyde 3-phosphate + NAD+  3-phosphoglycerate + NADH + H+
B. Acetaldehyde + NADH + H+  ethanol + NAD+
Sum: Glyceraldehyde 3-phosphate + acetaldehyde  3-phosphoglycerate + ethanol
A. Glyceraldehyde 3-phosphate dehydrogenase (3-磷酸甘油醛脱氢酶), type B
B. Alcohol dehydrogenase (乙醇脱氢酶), type A
There is no net change in the concentration of NAD+ + NADH.
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P.445
2). Vitamin B2 and Flavin coenzymes
异咯嗪
(维生素B2和黄素辅酶)
Riboflavin, or vitamin B2
(核黄素,维生素B2)
FMN
Lack of riboflavin causes angular
stomatitis, cheilosis, glossitis,
seborrhea, photophobia, etc.
FAD
Flavin coenzymes
Adenine
(腺嘌呤)
FMN (Flavin mononucleotide,
黄素单核苷酸)
FAD (Flavin adenine dinucleotide,
黄素腺嘌呤二核苷酸)
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How does a flavin coenzyme work?
异咯嗪
riboflavin
H = H+ + e
Flavoproteins/flavoenzymes
(黄素蛋白) are enzymes that
catalyze oxidation-reduction
reactions using either FMN
or FAD as coenzyme.
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P.446
FAD
FMN
Oxidized form
(氧化态)
max = 450 nm
(yellow)
Redox states of FAD and FMN
(FAD和FMN的氧化还原态)
+H+, e-
FADH
FMNH
Semiquinone form
+H+, e-
max = 570 nm – blue (at neutral pH)
max = 490 nm – red (at higher pH)
pKa = 8.4
FADH2
FMNH2
Reduced form
(还原态)
(colorless)
Flavin coenzymes:
– Are tightly bound to the enzyme
– Exist in any of three different redox states
– Change color according to the switch of the redox states
– Participate in one-electron and two-electron transfer reactions
– Work with different electron acceptors and donors (e.g. NAD) 28
III. Water-soluble vitamins
29
P.447
3). Pantothenic acid and coenzyme A
(泛酸和辅酶A)
III. Water-soluble vitamins
Reactive –SH group
-巯基乙胺
泛酸
Vitamin B5
Pantothenic acid (vitamin B5) is essential for metabolism of fat,
protein and carbohydrates in the citric acid cycle and other pathways.
30
The two major functions of CoA
Activation of acyl groups for transfer by nucleophilic attack
(活化酰基)
III. Water-soluble vitamins
H3C
C
O
O
O
OR1
CoA
H3C
C
S
H2O
CoA
H3C
C
OH
Activation of the -H of the acyl group for abstraction as a
proton (活化酰基上的-H )
O
H3C
C
O
O
SCoA
-
H2C
C
SCoA
H3C
C
O
H2
C
C
SCoA
O
B:
H3C
C
SCoA
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P.461







Vitamin C: L-ascorbic acid (抗坏血酸)
The only water-soluble vitamin that is unrelated to coenzymes
(与辅酶无关)
Simplest chemical structure of all the vitamins (结构最简单)
Physiological functions only presented by the L-form
Widely distributed in animals and plants, but humans are unable
to synthesize it.
Acidity (酸性)
Strong reducing agent (强还原剂) – a powerful antioxidant
acting like a cop in the body
Medical use: A panacea (万能药)?






Treatment and prevention of scurvy (坏血病)
Brain and nervous system
Mobilizing iron in the body
Preventing anemia (防止贫血)
Ameliorating allergic responses (改善变态反应)
Stimulating immune system (刺激免疫系统)
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III. Water-soluble vitamins
HO
HOH2C
H
5
HO
O
1
O
4
H
-2H
H
O
O
HOH2C
+2H
2
H
3
HO
OH
L-Ascorbic acid
(Vitamin C)
O
O
Dehydro-L-ascorbic acid
(脱氢抗坏血酸)
33
P.450
Vitamin B12: Cobalamin (钴胺素)


Vitamin B12 = cobalamin (钴胺素)
Two active coenzyme forms
III. Water-soluble vitamins
 5’-Deoxyadenosylcobalamin (5’-脱氧腺苷钴胺素)
 Methylcobalamin (甲基钴胺素)

Major functions:
 Preventing pernicious anemia (恶性贫血)
 Forming red blood cells
 Developing nerve cells

Uniqueness of vitamin B12:






The last to be discovered: 1948
4 Nobel prizes awarded: 1934, 1957, 1964, 1965
Containing a metal (Co)
Lowest nutritional requirement: 3 g/day
Sources: produced by only a few microorganisms
Dependent on an intrinsic factor -- a stomach-made
glycoprotein required for the absorption of B12
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5’-deoxyadenosylcobalamin (5’-脱氧腺苷钴胺素)
-CH3: methylcobalamin
(甲基钴胺素)
-CN: cyanocobalamin
(氰钴胺素)
-OH: hydroxocobalamin
(羟基钴胺素)
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Nobel Laureates and Their Work with Vitamin B12
Year
Winner(s)
Nobel Prize
Contribution
1934
Whipple, Minot,
Murphy
Physiology/Medicine
“for their discoveries
concerning liver therapy in
cases of anaemia”
1957
Todd
Chemistry
“for his work on nucleotides
and nucleotide co-enzymes”
Chemistry
“for her determinations by
X-ray techniques of the
structures of important
biochemical substances”
Chemistry
“for his outstanding
achievements in the art of
organic synthesis”
1964
1965
Hodgkin
Woodward
36
Reactions with B12 coenzymes involved
III. Water-soluble vitamins

Intramolecular rearrangements (分子内重排)
 Reductions of ribonucleotides to
deoxyribonucleotides (in certain bacteria) (核
苷酸还原成脱氧核苷酸)
 Methyl group transfers (甲基转移)
Methylcobalamin is a cofactor for methionine synthase involved in the
conversion of the amino acid homocysteine into methionine.
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P.464
IV. Metal Ions as Cofactors
1. About 1/3 of enzymes require metal ions for catalytic
activity (近1/3的酶需要金属离子)
2. Enzymes containing different metal ions:







Fe – e.g. cytochrome oxidase (细胞色素氧化酶)
Cu – e.g. ascorbic acid oxidase (抗坏血酸氧化酶)
Zn – e.g. carboxypeptidase (羧肽酶)
Ni – e.g. urease (尿酶)
Mo – e.g. nitrogenase (固氮酶)
Se – e.g. glutathion peroxidase (谷胱苷肽过氧化物酶)
Mn, V, etc.
3. These enzymes can be divided into two groups:
 Metalloenzymes (金属酶) – Tightly bound
e.g. tyrosinase (酪氨酸酶)
 Metal-activated enzymes (金属激活酶) – Loosely bound
e.g. carboxypeptidase A (羧肽酶A)
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The functions of metal ions
 Orienting the substrate for reaction (帮助底物分子定位)
 Stabilizing the charged reaction transition states (稳定带电荷的反应过渡态)
 Mediating oxidation-reduction reactions by reversible changes in the metal
ion’s oxidation state (通过氧化态的可逆变化来参与氧化还原反应)
Carboxypeptidase A
Zn
Thermolysin from Bacillus cereus
Ca – for structural stability
Zn – for enzyme activity
Loss of Zn  loss of activity
Can be replaced by divalent metal ions (Zn2+, Co2+, Ni2+)
39