Biological Oxidation Flashcards
1
Q
What is biological oxidation
A
Biological oxidation is that oxidation which occurs in biological systems to produce energy.
2
Q
Mention ways in which oxidation can occur in order of decreasing prevalence.
A
- Removal of Electrons (most common)
- Addition of Oxygen (common)
- Removal of Hydrogen (least common)
3
Q
Define oxidation and reduction chemically
A
oxidation is defined as the removal of electrons and
reduction as the gain of electrons.
4
Q
Why is oxidation always accompanied by reduction?
A
Electrons are not stable in the free state hence their removal from a substance (oxidation) must be accompanied by their acceptance by another substance (reduction)
5
Q
Redox reactions always involve pairs of compounds.
A pair of this type is referred to as ………………..
A
A redox pair or redox system or redox couple
6
Q
What is the esssential difference between two components of a redox system?
A
the number of electrons they contain
7
Q
The more electron-rich component of a redox system is called the ……, while the other one is referred to as the……..?
A
The more electron-rich component is called the reduced form of the compound concerned, while the other one is referred to as the oxidized form.
8
Q
The position of a system within one of these series is established by ……..?
A
Its redox potential
9
Q
What exactly is redox potential?
A
It is the affinity of a substance to accept electrons i.e. it is the potential for a substance to become reduced.
10
Q
How are electrons transferred with respect to the redox potential of substances?
A
Electrons are transferred from substances with low Redox potential to substances with higher redox potential, It is an energy yielding process.
11
Q
What does the amount of energy liberated depend on?
A
the Redox Potential difference between the electron donor and acceptor.
12
Q
Mention 5 quick facts about redox potential (E′o)
A
- It can be more negative or more positive than a reference potential.
- In addition, E’o depends on the concentrations of the reactants and on the reaction conditions
- In redox series , the systems are arranged according to their increasing redox potentials.
- Electrons flow from electronegative redox couple to more electropositive system.
- Spontaneous electron transfers are only possible if the redox potential of the donor is more negative than that of the acceptor.
13
Q
What are enzymes involved in oxidation and reduction called?
Mention four groups into which the aforementioned enzymes are classified
A
They are called oxido-reductases and are classified into four groups:
- oxidases,
- dehydrogenases (anaerobic dehydrogenases),
- hydroperoxidases
- oxygenases.
14
Q
What are oxidases?
A
These are enzymes that use oxygen as a hydrogen acceptor. They catalyze the removal of hydrogen from a substrate using oxygen as a hydrogen acceptor forming water.
The exceptions are uricase and monoamine oxidase that form hydrogen peroxide.
15
Q
Name the reaction products of oxidases
A
Water, hydrogen peroxide
16
Q
What are hemoproteins?
A
Hemoproteins are proteins linked to a nonprotein, iron-bearing component.
17
Q
What hemoprotein is an oxidase
A
Cytochrome oxidase
18
Q
List four features of cytochrome oxidase
A
- It has the typical heme prosthetic group present in myoglobin, hemoglobin, and other cytochromes.
- It is the terminal component of the chain of
respiratory carriers found in mitochondria. - It contains two molecules of heme, each having one Fe atom that oscillates between Fe3+ and Fe2+ during oxidation and reduction.
- Two atoms of Cu are present, each associated with a heme unit.
19
Q
What is an alternative name for cytochrome a3?
A
Cytochrome oxidase
20
Q
Name the complex formed from the combination of cytochromes a and a3
A
cytochrome aa3.
21
Q
What is the terminal component of the chain of respiratory carriers found in the mitochondria?
A
cytochrome oxidase.
22
Q
Mention inhibitors of cytochrome oxidase
A
carbon monoxide, cyanide, and hydrogen sulfide
23
Q
What is the effect of the inhibition of cytochrome oxidase?
A
Cytochrome oxidase is a key molecule in aerobic respiration and without a properly functioning enzyme can lead to poisoning, cell death by preventing cellular respiration.
24
Q
What are FMN and FAD formed from?
A
FMN and FAD are formed in the body from the vitamin, riboflavin-B2
25
Mention features of flavoprotein oxidases
1. Flavoprotein enzymes contain flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD) as prosthetic groups.
2. FMN and FAD are usually tightly —but not covalently— bound to their respective apoenzyme proteins.
26
Mention examples of flavoprotein
- L-amino acid oxidase
- xanthine oxidase
- aldehyde dehydrogenase
27
What are aerobic dehydrogenases?
They are enzymes (usually a metalloflavoenzyme) catalyzing the transfer of hydrogen from some metabolite to oxygen, forming hydrogen peroxide in the process.
28
Give an account on the examples of flavoprotein enzymes
L-amino acid oxidase, an FMN-linked enzyme found in kidney with general specificity for the oxidative deamination of the naturally occurring L-amino acids.
xanthine oxidase, which contains molybdenum and plays an important role in the conversion of purine bases to uric acid.
aldehyde dehydrogenase, an FAD-linked enzyme present in mammalian livers, which contains molybdenum and nonheme iron and acts upon aldehydes.
29
What are dehydrogenases?
These enzymes cannot use oxygen as a hydrogen acceptor. They are therefore called Anaerobic dehydrogenases.
30
Outline the main functions of dehydrogenases
1. Transfer of hydrogen from one substrate to another in a coupled oxidation-reduction reaction.
2. They are components of the electron transport chain.
31
What do dehydrogenases use as hydrogen carriers?
NAD+, NADP+ or both
32
What vitamin is NAD+ and NADP+ formed from?
Niacin, or vitamin B3
33
Mention features of dehydrogenases
They use nicotinamide adenine dinucleotide (NAD+) or nicotinamide adenine dinucleotide phosphate (NADP+), or both, as hydrogen carriers
The coenzymes are reduced by the specific substrate of the dehydrogenase and reoxidized by a suitable electron acceptor.
NAD-linked dehydrogenases catalyze oxidoreduction reactions in the oxidative pathways of metabolism, particularly in glycolysis, in the citric acid cycle, and in the respiratory chain of mitochondria.
The flavin groups associated with these dehydrogenases are similar to FMN and FAD occurring in oxidases.
They are generally more tightly bound to their apoenzymes than are the nicotinamide coenzymes.
Most of the riboflavin-linked dehydrogenases are concerned with electron transport in (or to) the respiratory chain.
34
What is use of reactions that enable one substrate to be oxidized at the expense of another?
They are particularly useful in enabling oxidative processes to occur in the absence of oxygen, such as during the anaerobic phase of glycolysis
35
Where are NADP-linked dehydrogenases found?
NADP-linked dehydrogenases are found characteristically in reductive syntheses, as in the extramitochondrial pathway of fatty acid synthesis, steroid synthesis and also in pentose phosphate pathway.
36
What is the difference between NADH linked dehydrogenase and other dehydrogenases?
NADH linked dehydrogenase acts as a carrier of electrons between NADH and the components of higher redox potential, while other dehydrogenases transfer reducing equivalents directly from the substrate to the respiratory chain.
37
Give examples of dehydrogenases that transfer reducing equivalents directly from the substrate to the respiratory chain.
- succinate dehydrogenase
- acyl-CoA dehydrogenase
- mitochondrial glycerol-3-phosphate dehydrogenase
38
All cytochromes are anaerobic dehydrogenases except ……….?
cytochrome oxidase (cyt a3) which is an oxidase and
cytochrome P450 that is mono-oxygenase (hydroxylase).
39
What are cytochromes?
The cytochromes are iron-containing hemoproteins in which the iron atom oscillates between Fe3+ and Fe2+ during oxidation and reduction.
40
How are cytochromes involved in the respiratory chain?
In the respiratory chain, they are involved as carriers of electrons from flavoproteins on the one hand to cytochrome oxidase on the other. Examples include cytochromes b, c1, c, a, and a3.
41
What cytochrome is found in the endoplasmic reticulum
cytochromes P450 and b5
42
……. has the highest redox potential?
Oxygen
43
……. has the lowest redox potential?
Hydrogen
44
What are hydroperoxidases?
These enzyme use hydrogen peroxide or an organic peroxide as substrate.
45
What are the two types of hydroperoxidases found in both animals and plants?
peroxidases and catalase.
46
What is the result of peroxidase accumulation??
Accumulation of peroxides can lead to generation of free radicals, which in turn can disrupt membranes and perhaps cause cancer and atherosclerosis.
47
How are peroxides produced in the cells?
either enzymatically or as byproducts of oxygen metabolism.
48
………. protect the body against harmful peroxides
Hydroperoxidases
49
A short note on peroxidases
These enzymes reduce peroxides using various electron acceptors.
Peroxidases are found in milk and in leukocytes, platelets, and other tissues involved in eicosanoid metabolism.
50
How is hydrogen peroxide reduced in reactions catalysed by peroxide?
In the reaction catalyzed by peroxidase, hydrogen peroxide is reduced at the expense of several substances that will act as electron acceptors, such as ascorbate, quinones, and cytochrome c.
51
What is the prosthetic group for glutathione peroxidase?
Selenium
52
How does glutathione peroxidase catalyzes the destruction of hydrogen peroxide and lipid hydroperoxides in erythrocytes, and other tissues?
Glutathione peroxidase (GPx) is an antioxidant enzyme that plays a crucial role in protecting cells from oxidative damage caused by hydrogen peroxide (H2O2) and lipid hydroperoxides (LOOHs). GPx catalyzes the reduction of these harmful reactive oxygen species (ROS) using reduced glutathione (GSH) as a co-substrate.
The mechanism of GPx action involves four steps:
Activation
GPx requires activation by reducing agents, such as thioredoxin or glutaredoxin, which reduce the active site selenocysteine (Sec) residue to selenol (-SeH).
Substrate recognition: H2O2 and LOOHs are recognized by GPx, which positions them within the enzyme's active site.
Catalytic cycle
GPx reduces the H2O2 or LOOHs by transferring electrons from GSH to the peroxide. During this process, the selenol group of Sec is oxidized to a selenenic acid (-SeOH), which is then reduced back to selenol by another molecule of GSH.
Termination
The GPx reaction terminates when the peroxide substrate is fully reduced to water or a corresponding alcohol.
In erythrocytes, GPx serves as the primary defense against H2O2 and LOOHs generated during hemoglobin metabolism. It prevents the accumulation of toxic lipid peroxides in the plasma membrane by reducing them to their corresponding alcohols. This function is essential for maintaining the integrity and function of erythrocytes, as well as for preventing the release of iron from hemoglobin, which can lead to oxidative damage.
In other tissues, GPx plays a similar role in protecting against oxidative stress. It is particularly important in tissues with high levels of polyunsaturated fatty acids, such as the liver and brain, where lipid peroxidation can lead to cellular damage and dysfunction. GPx activity is also essential for regulating the redox balance in cells, which is critical for many cellular processes, including cell signaling, metabolism, and apoptosis.
53
Where are catalases found?
Catalase is found in;
- blood
- bone marrow
- mucous membranes
- kidney, and liver.
54
What is the function of catalase?
Its function is assumed to be the destruction of hydrogen peroxide formed by the action of oxidases
55
What are peroxisomes?
a small organelle present in the cytoplasm of many cells, and in many tissues, including liver, which contains the reducing enzyme catalase and usually some oxidases.
56
Describe the mechanism of action of peroxidase.
1. Catalase process peroxidase activity
2. They also use one molecule of hydrogen peroxide as a substrate electron donor and another molecule of hydrogen peroxide as an oxidant or electron acceptor.
2H2O2 + catalase---------→ 2H2O + O2
57
How is hydrogen peroxide is continuously produced?
Hydrogen peroxide is continuously produced by the action of aerobic dehydrogenases and some oxidases.
However, mitochondrial and microsomal electron transport systems as well as xanthine oxidase must be considered as additional sources of Hydrogen peroxide.
58
How is hydrogen peroxide removed?
It is removed by the action of peroxidases and catalases to protect cells against its harmful effects.
59
What are oxygenases?
These enzymes catalyze direct incorporation(addition) of oxygen atom or molecule into organic substrate.
They catalyze direct incorporation of the two atoms of oxygen molecule into substrate.
60
What are two classes of. Oxygenases?
dioxygenases
mono-oxygenases.
61
Which dioxygenase participates in the catabolism of tryptophan?
tryptophan-2,3-dioxygenase (a heme enzyme)
62
Describe the mechanism of actually of monooxygenases
Monooxygenases incorporate one oxygen atom as a hydroxyl group into a substrate and the other is reduced to water.
These include the pyridine nucleotides, flavins, ferredoxins, hydroquinones, ascorbate, and others.
63
Mention requirements of Monooxygenases
- Molecular oxygen
- Presence of a co-substrate capable of donating a pair of electrons to reduce the second atom of oxygen to water.
64
Mention examples of monooxygenases
- Phenylalanine
- Hydroxylase
- Tyrosinase
- Cytochromes p450
These are monooxygenases important for the detoxification of many drugs & for the hydroxylation of steroids.
65
According to their intracellular localization Cytochromes P450 may be?
Microsomal cytochrome P450: mainly in the microsomes of liver cells (about 14% of the microsomal fraction of liver cells).
Or
Mitochondrial cytochrome P450: in mitochondria of many tissues but it is particularly abundant in liver and steroidogenic tissues as adrenal cortex, testis, ovary, placenta and kidney.
66
Where are mitochondrial cytochrome P450 systems found
In steroidogenic tissues such as adrenal cortex, testis, ovary, and placenta.
They are concerned with the biosynthesis of steroid hormones from cholesterol.
67
What are cofactors?
Cofactors are additional non ‑ protein component required by enzymes to carry out its catalytic functions.
A cofactor is a non-protein molecule that supports a biochemical reaction. Cofactors can take the form of metal ions, organic substances or other molecules with beneficial characteristics not typically present in amino acids.
68
Cofactors are divided into two groups; namely ……..
organic and inorganic cofactors.
69
What is a coenzyme?
A coenzyme is an organic cofactor that can bind within enzymes that require its function to catalyze a biochemical reaction.
They transport chemical groups between enzymes or from enzyme to substrate or product.
70
What is a prosthetic group?
A coenzyme that is covalently bound to the enzyme protein is called a prosthetic group.
71
Outline 4 functions of coenzymes
1. Coenzymes acts as intermediate carriers of transferred electrons or functional groups in a reaction.
2. Their function is usually to accept atoms or groups from a substrate and to transfer them to other molecules.
3. They are less specific than are enzymes and the same coenzyme can act as such in a number of different reactions.
4. The coenzymes are also attached to the protein at a different but adjacent site so as to be in a position to accept the atoms or groups which are removed from the substrate.
72
What is the chief function of tetrahydrofolic acid
The chief function of tetrahydrofolic acid is expressed as a carrier of formate and it is used in the synthesis of purines and pyrimidines.
73
What is the chief function of pyridoxal phosphate (B6-PO4)?
The chief function of pyridoxal phosphate (B6-PO4) is involved in transamination reactions.
74
What is the chief function of CoA?
The chief function of CoA is to carry acyl groups and they are used in the oxidative decarboxylation of pyruvic acid and synthesis of fatty acids and acetylation.
75
What do NAD and NADP coenzymes function as in dehydrogenation reactions?
hydrogen acceptors
76
NAD and NADP are derived from?
B3-vitamin, nicotinic acid.
77
Outline uses of NAD and NADP in biological systems
1. NAD and NADP act as coenzymes for many dehydrogenases where they are involved in transfer of hydrogen, causing either oxidation or reduction of the substrates.
2. Typically enzymes involved in anabolic pathways that create large molecules use NADPH, while enzymes involved in the breakdown of molecules use the analog NADH.
3. They can also contribute to storage of energy when the oxidized form, NAD+ or NADP+, gains two electrons to become NADH or NADPH, which are the respective reduced forms.
4. They release the stored energy when the reverse reaction occurs and they become oxidized, ultimately by oxygen.
5. It is often stated that these compounds are electron carriers because they accept electrons (become reduced) during catabolic steps in the breakdown of organic molecules such as carbohydrates and lipids.
6. Then, these reduced coenzymes can donate these electrons to some other biochemical reaction normally involved in a process that leads to the synthesis of ATP.
78
Nicotinamide Adenine Dinucleotide in its oxidized state is called ……….?
NAD+
79
Reduced Nicotinamide Adenine Dinucleotide is referred to as?
NADH
80
What is the function of niacin in Nicotinamide Adenine Dinucleotide structure
Niacin provides the organic ring structure(nicotinamide) that will directly participate in the transfer of a hydrogen atom and 2 electrons(a hydride ion).
81
NAD+ is often found in conjunction with a ………. enzyme.
"dehydrogenase"
82
Describe a dehydrogenase reaction.
A dehydrogenase reaction removes two hydrogen atoms; one as a hydride (:H-) (a hydride is a hydrogen atom with 2 electrons) and one as a hydrogen cation (H+) (and of course, a hydrogen cation has no electrons).
The hydride bonds with NAD+ and creates a reduced compound of Nicotinamide Adenine Dinucleotide (NADH). The second hydrogen atom (H+) is released into solution.
83
What type of coenzymes are commonly called flavoproteins?
FMN and FAD
They are hydrogen transferring coenzymes associated with hydrogenases.
84
What does the coenzyme part of flavoproteins contain
The B2-vitamin, riboflavin
85
What’s a contrasting feature between flavoproteins, and NAD or NADP
In contrast to NAD or NADP, the coenzymes of flavoproteins are more tightly bound to the apoenzyme. As a result they cannot be separated by dialysis.
86
What is FAD converted to on reduction?
On reduction of FAD by addition of two H-atoms donated by a substrate, it is converted to FADH2; the substrate is therefore oxidised.
87
An example of FAD containing enzyme is?
Succinate dehydrogenase occurring in the Krebs’ cycle.
88
In the oxidation of succinic acid to fumaric acid by succinate dehydrogenase, where is the hydrogen accepted by FAD transferred to.
The hydrogen accepted by FAD is transferred to the electron transport chain for generation of ATP.
89
What is the role of riboflavins in FAD?
Riboflavin provides the ring structures that will directly participate in the transfer of two hydrogen atoms (each with one electron this time).
90
What’s a similarity between NAD and FAD?
Similar to NAD, FAD works in association with a "dehydrogenase" enzyme.
91
Succinic acid is oxidized to fumaric acid by succinate dehydrogenase, an FAD containing enzyme. Describe the effect on FAD
The reaction removes two hydrogen atoms; each a proton with one electron.
Both hydrogen atoms bond with FAD.
This reaction does not release an H+ into solution like the reduction of NAD does.
Flavin adenine dinucleotide in the oxidized form (FAD) accepts two hydrogen atoms (each with one electron) and becomes FADH2.
92
What are the constituents of CoA?
a β-mercaptoethylamine group linked to the vitamin pantothenic acid through an amide linkage and 3'-phosphorylated ADP.
93
What are other names for coenzyme A?
CoASH or CoA
94
What are the requirements for CoA biosynthesis in humans?
- Cysteine
- Pantothenate (vitamin B5)
- Adenosine triphosphate (ATP).
95
What distinguishes acetyl coA from coenzyme A
Acetyl-CoA is Coenzyme A in which the H atom in the thiol group has been replaced by an acetyl group.
96
Coenzyme A being a thiol can react with carboxylic acids to form ………..?
Thioesters, thus functioning as an acyl group carrier. It therefore assists in transferring fatty acids from the cytoplasm to mitochondria.
97
A molecule of coenzyme A carrying an acyl group is also referred to as ……….?
Acyl-CoA
98
What’s TPP, thiamine pyrophosphate?
Thiamine pyrophosphate is a coenzyme involved in transfer of aldehyde groups (-C-H) groups like acetaldehyde and glycol aldehyde.
TPP is involved in oxidative decarboxylation of pyruvic acid and α-ketoglutaric acid.
99
What vitamin does TPP contain?
It contains Thiamine, a vitamin of B-group.
100
…………… of the coenzyme molecule accepts the aldehyde group and transfers it to an acceptor via other coenzymes? And what are examples of such “other coenzymes”
The thiazole group
Lipoic acid and coenzyme A are the examples
101
An example of an enzyme complex involving TPP,Lipoic acid and coenzyme A is ……….?
The Pyruvate decarboxylase.
102
What’s pyridoxal phosphate?
Pyridoxal phosphate is a coenzyme associated with transaminases which catalyse transfer of amino groups from amino acids to keto acids.
103
Describe the transamination reaction in which pyridoxal phosphate is associated with
In this transfer process, Pyridoxal phosphate acts as the acceptor of the amino group and is converted to pyridoxamine phosphate (PAM).
PAM can react with a keto acid to produce an amino acid.
Pyridoxal phosphate and pyridoxamine phosphate remain bound to the protein part of the transaminase enzyme during these transfer of amino group.
104
The aldehyde group is the reactive group of the coenzyme which binds to the amino acid forming a ………….?
Schiff base.
105
Lipoic acid is involved in oxidative decarboxylation reactions, such as ……………
those catalysed by pyruvic decarboxylase or α-keto glutarate decarboxylase
106
Biotin is bound to enzymes involved in ……………
carboxylation reactions.
Biotin is a coenzyme for several reactions involving CO2 fixation into various compounds
107
What is the role of biotin in carboxylation reactions?
Biotin acts as the carrier of CO2.
108
Mention one example of a biotin bound enzyme involved in carboxylation.
An example is pyruvate carboxylase which adds a CO2 molecule to pyruvic acid forming oxalacetic acid.
Pyruvate carboxylase (PC) is a biotin-containing enzyme that catalyzes the formation of oxaloacetate in the presence of an allosteric activator.
