Biological Oxidation Flashcards
What is biological oxidation
Biological oxidation is that oxidation which occurs in biological systems to produce energy.
Mention ways in which oxidation can occur in order of decreasing prevalence.
- Removal of Electrons (most common)
- Addition of Oxygen (common)
- Removal of Hydrogen (least common)
Define oxidation and reduction chemically
oxidation is defined as the removal of electrons and
reduction as the gain of electrons.
Why is oxidation always accompanied by reduction?
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)
Redox reactions always involve pairs of compounds.
A pair of this type is referred to as ………………..
A redox pair or redox system or redox couple
What is the esssential difference between two components of a redox system?
the number of electrons they contain
The more electron-rich component of a redox system is called the ……, while the other one is referred to as the……..?
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.
The position of a system within one of these series is established by ……..?
Its redox potential
What exactly is redox potential?
It is the affinity of a substance to accept electrons i.e. it is the potential for a substance to become reduced.
How are electrons transferred with respect to the redox potential of substances?
Electrons are transferred from substances with low Redox potential to substances with higher redox potential, It is an energy yielding process.
What does the amount of energy liberated depend on?
the Redox Potential difference between the electron donor and acceptor.
Mention 5 quick facts about redox potential (E′o)
- 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.
What are enzymes involved in oxidation and reduction called?
Mention four groups into which the aforementioned enzymes are classified
They are called oxido-reductases and are classified into four groups:
- oxidases,
- dehydrogenases (anaerobic dehydrogenases),
- hydroperoxidases
- oxygenases.
What are oxidases?
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.
Name the reaction products of oxidases
Water, hydrogen peroxide
What are hemoproteins?
Hemoproteins are proteins linked to a nonprotein, iron-bearing component.
What hemoprotein is an oxidase
Cytochrome oxidase
List four features of cytochrome oxidase
- 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.
What is an alternative name for cytochrome a3?
Cytochrome oxidase
Name the complex formed from the combination of cytochromes a and a3
cytochrome aa3.
What is the terminal component of the chain of respiratory carriers found in the mitochondria?
cytochrome oxidase.
Mention inhibitors of cytochrome oxidase
carbon monoxide, cyanide, and hydrogen sulfide
What is the effect of the inhibition of cytochrome oxidase?
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.
What are FMN and FAD formed from?
FMN and FAD are formed in the body from the vitamin, riboflavin-B2
Mention features of flavoprotein oxidases
- Flavoprotein enzymes contain flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD) as prosthetic groups.
- FMN and FAD are usually tightly —but not covalently— bound to their respective apoenzyme proteins.
Mention examples of flavoprotein
- L-amino acid oxidase
- xanthine oxidase
- aldehyde dehydrogenase
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.
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.
What are dehydrogenases?
These enzymes cannot use oxygen as a hydrogen acceptor. They are therefore called Anaerobic dehydrogenases.
Outline the main functions of dehydrogenases
- Transfer of hydrogen from one substrate to another in a coupled oxidation-reduction reaction.
- They are components of the electron transport chain.
What do dehydrogenases use as hydrogen carriers?
NAD+, NADP+ or both
What vitamin is NAD+ and NADP+ formed from?
Niacin, or vitamin B3
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.
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
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.
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.
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
All cytochromes are anaerobic dehydrogenases except ……….?
cytochrome oxidase (cyt a3) which is an oxidase and
cytochrome P450 that is mono-oxygenase (hydroxylase).
What are cytochromes?
The cytochromes are iron-containing hemoproteins in which the iron atom oscillates between Fe3+ and Fe2+ during oxidation and reduction.
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.
What cytochrome is found in the endoplasmic reticulum
cytochromes P450 and b5
……. has the highest redox potential?
Oxygen
……. has the lowest redox potential?
Hydrogen