Bioinorganic Chemistry MCQ Quiz in मराठी - Objective Question with Answer for Bioinorganic Chemistry - मोफत PDF डाउनलोड करा

Concept:-

• Carboxypeptidase-A is a metalloenzyme that requires a zinc ion (Zn²⁺) as a cofactor for its catalytic activity. The coordination of the zinc ion by specific amino acid residues is essential for the enzyme's function.
• The active site of Carboxypeptidase-A is shown below:

• Carboxypeptidase A (CPA) contains a zinc (Zn²⁺) metal center in a tetrahedral geometry with amino acid residues in close proximity around zinc to facilitate catalysis and binding.
• Out of the 307 amino acids bonded in a peptide chain, the following amino acid residues are important for catalysis and binding; Glu-270, Arg-71, Arg-127, Asn-144, Arg-145, and Tyr-248.
• The above figure illustrates the tetrahedral zinc complex active site with the important amino acid residues that surround the complex.
• The zinc metal is a strong electrophilic Lewis acid catalyst which stabilizes a coordinated water molecule as well as stabilizes the negative intermediates that occur throughout the hydrolytic reaction.
• Stabilization of both the coordinated water molecule and negative intermediates are assisted by polar residues in the active site which are in close proximity to facilitate hydrogen bonding.
• The active site can be characterized into two sub-sites denoted as S₁’ and S₁. The S₁’ sub-site is the hydrophobic pocket of the enzyme, and Tyr-248 acts to ‘cap’ the hydrophobic pocket after substrate or inhibitor is bound (SITE). 
• The hydrogen bonding from the hydroxyl group in Tyr-248 facilitates this conformation due to interaction with the terminal carboxylates of substrates that bind. Substantial movement is required for this enzyme and induced fit model explains how this interaction occurs.

Explanation:-

• Carboxypeptidase A (CPA) contains a zinc (Zn²⁺) metal center in a tetrahedral geometry with amino acid residues in close proximity around zinc to facilitate catalysis and binding. 
• The zinc metal is a strong electrophilic Lewis acid catalyst which stabilizes a coordinated water molecule as well as stabilizes the negative intermediates that occur throughout the hydrolytic reaction.
• Thus, statement A is correct.
• Option B, which suggests that the substitution of the Zn²⁺ ion by Co²⁺ ion renders the enzyme inactive, is incorrect. While the substitution of Zn²⁺ by Co²⁺ may impact the enzyme's activity, it does not necessarily render it completely inactive. The activity of the enzyme may be altered, but it can still retain some functionality.
• Statment C is correct as two histidine nitrogen atoms, glutamate oxygen atom(s), and a water molecule coordinate to a Zn²⁺ ion in carboxypeptidase-A.
• Option D, which states that three histidine nitrogen atoms and a water molecule coordinate to a Zn²⁺ ion, is incorrect. Carboxypeptidase-A typically coordinates with two histidine nitrogen atoms, a glutamate oxygen atom(s), and a water molecule to the Zn²⁺ ion. The two histidines and the glutamate residue provide important ligands for binding and activating the zinc ion.

Conclusion:-

Hence, the correct statements about carboxypeptidase-A are A and C only. 

Explanation: 

Met-hemoglobin is a form of hemoglobin that contains ferric [Fe³⁺] iron. Met-hemoglobin cannot bind oxygen, unlike oxyhemoglobin. In human blood a trace amount of methemoglobin is normally produced spontaneously, but when present in excess the blood becomes abnormally dark bluish brown & disorder is known as Met-hemoglobinemia or blue skin syndrome.

Concept:

Myoglobin is a protein located primarily in the striated muscles of vertebrates. MB is the gene encoding myoglobin in humans. It encodes a single polypeptide chain with one oxygen binding site. Myoglobin contains a heme prosthetic group that can reversibly bind to oxygen.

Explanation:

→  The structure of myoglobin is similar to the structure of one of the β subunits of hemoglobin. Myoglobin and hemoglobin are both part of the globin family; a family of heme-containing globular polypeptides with eight α-helices in their protein fold. Myoglobin contains only one subunit of globin, while hemoglobin has four subunits.

→ The iron (Fe)-containing heme group allows myoglobin to reversibly bind to O₂ . Heme is a large, aromatic porphyrin ring with four pyrrole nitrogens bound to a ferrous (Fe(II)) ion at the center. The nitrogens from the porphyrin ring and a Histidine imidazole serve as ligands for the Fe(II) metal center.

→ The heme Fe is bound to the myoglobin polypeptide through the proximal histidine residue. The iron ion has six coordination sites: four equatorial sites are occupied by pyrrole nitrogens of heme, and one axial site is occupied by a proximal histidine residue.2 The remaining axial coordination site is available for binding a O₂ molecule.

Conclusion: The correct answer is option 1.

Explanation:

Metallobiomolecules is a generic term for a molecules that contains a metal ion cofactor.

→ Transferrin 

The molecule has a beta alpha structure of similar topology to human lactoferrin and is composed of two homologous lobes that each bind a single ferric ion. Each lobe is further divided into two dissimilar domains, and the iron-binding site is located within the interdomain cleft.

→ Siderophores 

It usually form a stable, hexadentate, octahedral complex preferentially with Fe³⁺ compared to other naturally occurring abundant metal ions, although if there are fewer than six donor atoms water can also coordinate.

→ Hydrogenases 

It catalyze the reversible conversion of molecular hydrogen to protons and electrons via a heterolytic splitting mechanism. The active sites of [NiFe] hydrogenases comprise a dinuclear Ni-Fe center carrying CO and CN^- ligands.

→ Hydroxylases

They are enzymes which add an hydroxyl group to organic compounds. This addition is the first step of aerobic oxidative degradation.

Secondary structure of Human phenylalanine hydroxylase catalytic domain. Fe⁺³ ion coordination site in Human phenylalanine hydroxylase catalytic domain 

→ Hameerythrin

The conformation of the hemerythrin subunit designated the hemerythrin fold, has four nearly parallel α-helical segments surrounding the pair of non-heme iron atoms, the oxygen binding site. These iron atoms are linked to the protein through specific histidine and tyrosine side chains

Conclusion: All the structures contains metal as co factor, so the correct answer is option 4.

Concept:

→ The main difference between oxyhemoglobin and deoxyhemoglobin is that the oxyhemoglobin is the form of haemoglobin loosely combined with oxygen whereas the deoxyhemoglobin is the form of hemoglobin that has released its bound oxygen. Furthermore, the oxyhemoglobin is bright red in color while the deoxyhemoglobin is purplish in color. 

→Oxyhemoglobin is the oxygen-bound form of hemoglobin. During respiration in the lungs, the hemoglobin component of the red blood cells is exposed to oxygen and loosely bound to it. The binding of oxygen into hemoglobin occurs at high pH, low carbon dioxide, and high-temperature conditions of the blood, which generally occurs inside the lungs. With the binding of the first oxygen molecule to the iron (II), the heme pulls the iron (II) into the porphyrin ring. This slight conformational shift encourages the binding of another three oxygen molecules to the hemoglobin. Ultimately, oxyhemoglobin contains four bound oxygen molecules in its fully saturated form. Therefore, oxyhemoglobin is considered to be in the relaxed (R) state of hemoglobin.  

→Deoxyhemoglobin is the hemoglobin that has released oxygen. The release of oxygen occurs at the metabolizing tissue due to the low pH, high carbon dioxide concentration, and low temperature. Deoxyhemoglobin is the tensed (T) state of hemoglobin due to the release of oxygen molecules.  

Explanation:

In ferrous hemoglobin and myoglobin, coordination numbers of five and six are associated with the high-spin (as in deoxy-hemoglobin) and low-spin (as in oxy-hemoglobin) states respectively.

In deoxyhemoglobin, the heme iron atom is in its high-spin paramagnetic form.

In oxyhemoglobin, the heme iron atom is in its low-spin diamagnetic form because oxygen molecules are bound to iron.

Conclusion: The correct answer is option 1.

Concept:

Ferredoxins are iron–sulfur proteins that mediate electron transfer in a range of metabolic reactions. 

→Ferredoxins are small proteins containing iron and sulfur atoms organized as iron–sulfur clusters. These biological "capacitors" can accept or discharge electrons, with the effect of a change in the oxidation state of the iron atoms between +2 and +3. In this way, ferredoxin acts as an electron transfer agent in biological redox reactions.

Explanation:

→Ferredoxins typically carry out a single electron transfer.

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However a few bacterial ferredoxins (of the 2[4Fe4S] type) have two iron-sulfur clusters and can carry out two-electron transfer reactions. Depending on the sequence of the protein, the two transfers can have nearly identical reduction potentials or they may be significantly different.

 

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→ Ferredoxins are one of the most reducing biological electron carriers. They typically have a midpoint potential of -420 mV. The reduction potential of a substance in the cell will differ from its midpoint potential depending on the concentrations of its reduced and oxidized forms. For a one-electron reaction, the potential changes by around 60 mV for each power of ten changes in the ratio of the concentration.

For example, if the ferredoxin pool is around 95% reduced, the reduction potential will be around -500 mV. In comparison, other biological reactions mostly have less reducing potential: for example the primary biosynthetic reductant of the cell, NADPH has a cellular redox potential of -370 mV (Eo= -320 mV).

Conclusion: The correct answer is option 4.

Explanation:

Hemoglobin is responsible for the red color of blood. Hemes are the basic unit for building hemoglobin. Hemes have the capability of binding oxygen and iron molecules.

In oxyhaemoglobin, Fe²⁺=[Ar]3d⁶

The blood cells are red because of the binding of oxygen and iron molecules. When oxygen and Fe bind together they exchange their orbitals bond and energy. During binding, the Fe molecule changes its oxygen state from +2 to +3 and shows π – π* Interligand transition.

Interligand π – π* transition refers to an electronic transition that occurs between two adjacent ligands in a complex molecule. In such a transition, an electron from a π orbital of one ligand is excited to an empty π* orbital of another ligand.

The absorption spectrum of oxyhemoglobin shows a peak at around 415 nm, which corresponds to the Soret band, due to the π-π* transition of the porphyrin ring of the heme group. The Soret band is responsible for the blue-green color of oxyhemoglobin.

This type of transition is commonly observed in coordination complexes containing conjugated ligands, such as aromatic compounds. The transition is usually initiated by absorption of light, which excites the electrons from the π orbital of the donor ligand to the π* orbital of the acceptor ligand.

The sharp high energy transition between t_2g​ and e_g​ levels of the d-electrons is responsible for the bright red colour of oxyhaemoglobin.

Conclusion: The correct answer is option 3.

Explanation:-

A. Ferritin is a protein that plays a crucial role in iron storage and sequestration. It binds with iron and stores it in a non-toxic form until needed by the body. Hence, the correct match for Ferritin is 5 - Iron storage.

• So, the correct match is A-5

B. In the human mitochondria, an adenosyl-transferase incorporates the organometallic group of coenzyme B₁₂. In all these enzymes, the bound B₁₂-derivatives engage (or are formed) in exceptional organometallic enzymatic reactions.

• So, the correct match is B-4

C. Cytochromes are proteins that contain heme groups and are involved in electron transport in the respiratory chain. Hence, the correct match for Cytochromes is 1- Electron transport.

• So, the correct match is C-1

D. Valinomycin is a cyclic peptide antibiotic that acts as an ionophore, transporting potassium ions across the cell membrane. Hence, the correct match for Valinomycin is 2- Ionophore.

• So, the correct match is D-2

Conclusion:-

• Therefore, the correct match for the compounds with their respective functions is A-5, B-4, C-1, and D-2. Option 2 is the correct answer.

Concept:

• Carboxypeptidase A usually refers to the pancreatic exopeptidase that hydrolyzes peptide bonds of the C-terminal residues. This enzyme is referred to as CPA1.
• The oxygen-evolving complex (OEC), a water-splitting complex, is the portion of photosystem II where photo-oxidation of water occurs during the light reactions of photosynthesis at chlorophyll.
• Hemerythrin is a non-heme protein responsible for oxygen transport in the marine invertebrate.

Explanation:

• The metalloprotein Carboxypeptidase A or CPA1 contains a zinc (Zn²⁺) metal center in a tetrahedral geometry with amino acid residues in close proximity around the zinc metal ion to facilitate catalysis and binding. Thus, A - IV.
• The metalloprotein core of the oxygen-evolving complex (OEC) contains both manganese (Mn) and calcium ion. Thus, B - I.
• The metalloprotein hemerythrin involves Fe in the +2 oxidation state. The uptake of O₂ by hemerythrin involves the two-electron oxidation of the diferrous center to produce a hydroperoxide (OOH−) complex. Thus, C - II.
• The metalloprotein coenzyme F-430 is a nickel protein. Thus, D - III. 

Conclusion:

• Hence, the correct answer is A - IV, B - I, C - II, and D - III.

Concept:

Hemocyanin:

• From the name, it may suggest that Hemocyanin contains heme and cyanide, but it contains neither.
• The metal atom present in Hemocyanin is Copper and the meaning of Hemocyanin is 'Blue Blood'.
• Hemocyanin is a copper-containing protein that carries oxygen in some invertebrates such as snails, squids, etc.
• The deoxy form of hemocyanin contains Cu in a +1 oxidation state and is colourless.
• The oxy form of hemocyanin contains Cu in +2 form and is bridged to oxygen via an O22--Cu2+ LMCT.

Explanation:

• Hemocyanin molecule has several subunits and binds oxygen cooperatively. 
• The active sites contain two Cu (I) ions 360pm apart for binding of one dioxygen cooperatively.
• Each Cu(I) ion is bound by three histidine residues.
• The dioxygen molecule oxidizes each Cu(I) ion to Cu(II) ion and itself is reduced to the peroxide ion O₂^2-. 
• The peroxide ion bridges two Cu²⁺ ions in theμ−η2:η2−O2−" id="MathJax-Element-8-Frame" role="presentation" style="position: relative;" tabindex="0">μ−η2:η2−O−2μ−η2:η2−O2− $\mu-\eta^2:\eta^2-O_2^-$ mode. The resonance Raman spectroscopy reveals the formulation of Cu-O-O-Cu linkage.
• The 'O-O' stretching frequency in Raman is 745-750 cm^-1, which suggests that the peroxide is not in a free state.
• The UV stretching frequency is about 350-580nm, for the LMCT occurring in oxyhemocyanin imparting it a blue colour.
• The two Cu(II) ions are coupled antiferromagnetically with the μ-O₂^2- ion being involved in a superexchange mechanism.
• As the ions are coupled, there is no magnetic moment observed. The molecule is hence diamagnetic.
• Hemocyanin is colourless in deoxygenated state and blue in an oxygenated state.
• The two states are represented below:

• From the diagram above it is clear that the coordination number of hemocyanin is 5 in oxygenated form.
• Hence, in oxyhemocyanin, the coordination number, mode of oxygen binding, colour and the net magnetic behaviour of copper ions, respectively are five, \(\mu-\eta^2:\eta^2-O_2^-\) blue and diamagnetic.