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Sulfur Assimilation

Sulfur Assimilation

Sulfur is an essential element in biological systems, contributing to synthesizing key biomolecules, including amino acids such as cysteine and methionine, and cofactors such as coenzyme A and biotin. Microorganisms primarily assimilate sulfur as sulfate (SO₄²⁻) from the environment, which must undergo a series of biochemical transformations before it can be incorporated into cellular components. As sulfate is highly oxidized, it must undergo assimilatory sulfate reduction to...

Preparation and Reactions of Sulfides

Preparation and Reactions of Sulfides

Sulfides are the sulfur analog of ethers, just as thiols are the sulfur analog of alcohol. Like ethers, sulfides also consist of two hydrocarbon groups bonded to the central sulfur atom. Depending upon the type of groups present, sulfides can be symmetrical or asymmetrical. Symmetrical sulfides can be prepared via an SN2 reaction between 2 equivalents of an alkyl halide and one equivalent of sodium sulfide.

Preparation and Reactions of Thiols

Preparation and Reactions of Thiols

Thiols are prepared using the hydrosulfide anion as a nucleophile in a nucleophilic substitution reaction with alkyl halides. For instance, bromobutane reacts with sodium hydrosulfide to give butanethiol.

Structure and Nomenclature of Thiols and Sulfides

Structure and Nomenclature of Thiols and Sulfides

Thiols and sulfides are sulfur analogs of alcohols and ethers, respectively, where the sulfur atom takes the place of the oxygen atom. Thus, thiols are generally represented as RSH, where R is an alkyl substituent and —SH is the functional group. On the other hand, in sulfides, the central sulfur atom is bonded to two hydrocarbon groups on either side. Depending upon the type of group, sulfides can be either symmetrical or asymmetrical. Both thiols and sulfides display a bent geometry,...

Redox Reactions

Redox Reactions

Redox reactions are vital biochemical processes that underpin energy metabolism in cells. These reactions involve the transfer of electrons between molecules, occurring in tandem as oxidation and reduction. Oxidation refers to the loss of electrons, while reduction denotes their gain. This coupling ensures the seamless flow of electrons through metabolic pathways. For example, in bacterial metabolism, glucose undergoes oxidation to carbon dioxide, while oxygen is simultaneously reduced to...

Redox Reactions

Redox Reactions

Oxidation-reduction or redox reactions involve the transfer of electrons from one molecule or atom to another. When an atom gains an electron, another atom must lose an electron, meaning oxidation and reduction must occur together. Since the redox occurs in pairs, the atom that gets oxidized is also called the reducing agent or reductant, and the atom that is reduced is also called the oxidizing agent or oxidant. A straightforward way to remember the definitions of oxidation and reduction is...

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Why does sulfite reductase employ siroheme?

Adrian M V Brânzanic¹, Ulf Ryde², Radu Silaghi-Dumitrescu³

¹Department of Chemistry, Babes-Bolyai University, Cluj-Napoca, Romania. rsilaghi@chem.ubbcluj.ro and Institute of Interdisciplinary Research in Bio-Nano-Sciences, Babes-Bolyai University, Cluj-Napoca, Romania.

Chemical Communications (Cambridge, England)

|November 7, 2019

Summary

Sulfite reductase (SiR) uses a unique siroheme cofactor, not heme, to efficiently channel electrons for sulfite reduction. This prevents side reactions, optimizing the enzyme's catalytic function.

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Area of Science:

Biochemistry
Enzyme mechanisms
Bioinorganic chemistry

Background:

Sulfite reductase (SiR) catalyzes the six-electron reduction of sulfite to sulfide.
The enzyme's active site features a unique siroheme cofactor and a [4Fe4S] cluster.
Siroheme is a modified heme derivative with distinct electronic properties.

Purpose of the Study:

To elucidate the functional advantage of siroheme over heme in sulfite reductase.
To understand the electronic factors governing electron transfer in SiR.

Main Methods:

Non-equilibrium Green's function (NEGF) methods.
Density functional theory (DFT) computations.
Comparative analysis of heme vs. siroheme in the SiR active site.

Main Results:

Replacing siroheme with heme inhibits direct charge-transfer pathways.
Siroheme facilitates efficient electron channeling to the catalytic iron center.
This selective electron transfer minimizes unproductive side reactions.

Conclusions:

Siroheme's unique structure is crucial for SiR's high catalytic efficiency.
The enzyme's design prevents electron delocalization, ensuring precise sulfite reduction.
Computational methods provide insights into the necessity of siroheme in SiR.