Heavy Metal Bhuvan

8/8/2019 Heavy Metal Bhuvan

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Heavy-metals & their effects

on Microorganisms

Submitted to: Mrs. Mamtha mamSubmitted to: Mrs. Mamtha mam

Department of MicrobiologyDepartment of Microbiology

Submitted by: Bhuvanesh SK

4th Semester M.Sc. Microbiology

DAYANANDA SAGAR COLLEGE OFBIOLOGICAL SCIENCES

(Shivage Malleshwara Hills, Kumaraswamy Layout,Bangalore-560 078)


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What is heavy metals ?

Heavy metals are natural components of the Earth's crust, with aspecific gravity that is at least 5 times the specific gravity of water &that has a relatively high density and is toxic or poisonous at lowconcentrations. E.g. mercury (Hg), cadmium (Cd), arsenic (As),chromium (Cr), thallium (Tl), and lead (Pb).

They cannot be degraded or destroyed

As trace elements, some heavy metals (e.g. copper, selenium, zinc)are essential to maintain the metabolism of the cell.

At higher concentrations they can lead to poisoning.

Essential Metals play an integral role in the life processes ofmicroorganisms; serve as micronutrients as components of variousenzymes and for regulation of osmotic pressure.

Nonessential metals have no biological role and are potentially toxicto microorganism; through the displacements of essential metals fromtheir native binding sites or through legend interactions.

At high levels, both essential and nonessential metals can damagecell membranes, alter specificity, disrupt cellular functions anddamage the structure of DNA.


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Metals in the environment can be divided

into two classes:

(1) Bioaviable (soluble, nonsorbed andmobile) and

(2) Nonbioavailable (precipitated,

complexed, and thus toxic to biological

systems).


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Heavy metals Sources

As Semiconductors, petroleum refining, wood preservatives, animal feed

additives, coal power plants, herbicides, volcanoes, mining andsmelting

Cu Electroplating industry, smelting and refining, mining, biosolids

Cd Geogenic sources, anthropogenic activities, metal smelting and refining,

fossil fuel burning, application of phosphate fertilizers, sewage sludge

Cr Electroplating industry, sludge, solid waste, tanneries

Pb Mining and smelting of metalliferous ores, burning of leaded gasoline,

municipal sewage, industrial wastes enriched in Pb, paints

Hg Volcano eruptions, forest fire, emissions from industries producing caustic

soda, coal, peat and wood burning

Se Coal mining, oil refining, combustion of fossil fuels, glass manufacturing

industry, chemical synthesis (e.g., varnish, pigment formulation)

Ni Volcanic eruptions, land fill, forest fire, bubble bursting and gas exchange

in ocean, weathering of soils and geological materials

Zn Electroplating industry, smelting and refining, mining, biosolids

Different sources of heavy metals


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Position of heavy metals in Mendeleef table

1. Macrobiogenetic elements

2. Microbiogenetic elements

3. Toxic elements

4. Elements with Chemoterapeutical effect


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Heavy Metal Toxicity

Effects of heavy metals can be drawn oni) disruptions on a vast assortment of metabolic processes;

ii) alterations on a balance of pro-oxidant and antioxidant systems;&

iii) competitions with nutrient trace elements for binding sites onessential metalloenzymes, receptors, metal-binding transporter andstorage systems.

e.g. Act as pro-oxidant which catalyzes the production ofperoxides and

enhances the subsequent formation ofhydroxy radicals and lipidperoxides, and form highly reactive oxygen species.

Act as anti-oxidatant result in inhibition of superoxide dismutase

(SOD) and catalase action

Interact with free sulfhydryl groups,and inhibit enzymes involved invarious metabolism;

Formation of free radicals, which interfere in protein, nucleic acid, and

energy metabolism


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Classification of naturally occurring metals

by toxicity

ORDER OF SENSITIVITY to metal ions of heterotrophic microorganism isAg+ Cu2+ = Ni2 > Ba2+ = Cr6+ = Hg2+ > Zn2+ = Na+ = Cd2+ = Pb2+

The 3 most pollutants heavy metals are Lead, Cadmium, and Mercury.


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Structural similarity of divalent metal cations (e.g. Mn2+, Fe2+, Co2+, Ni2+,Cu2+ and Zn2+) ;chromate resembles that of sulfate, and arsenate to PO4.

Two types of uptake systems for metal ions.

1) fast, unspecific, and driven by the chemiosmotic gradient across the cytoplasmicmembrane of bacteria. Since this mechanism is used by a variety ofsubstrates, it is constitutively expressed.

2) high substrate specificity, is slower, often uses ATP hydrolysis as the energysource and is only produced by the cell in times of need, starvation or a specialmetabolic situation; they are inducible.

The governing sorption mechanism's for heavy metals in microorganisms dependlargely on:

1. Cell viability (passive or active sorption),2. Microbial species (eucaryote: gram-positive/negative, archaebacteria),

3. Heavy metal levels in solution (essential trace element or toxic compound),

4. Chemical composition and pH of the liquid phase,

5. Temperature, and

6. Growth phase of the microorganism.

Heavy Metal Toxicity Cont..


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Ether and ester linkage


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M M

M

M2+

_

__

__

__M2+

Metal-bindingproteins

e

M(red)

H2S

HPO42

+M2+

M(ox.)

M(ox.)

M(red)

M2+

M-CH2

MetalprecipitationEnzymatic

Transformation

Means by which bacteria react in presence of Metals(M2+) in medium


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Heavy Metal Resistance

Because metal ions cannot be degraded ormodified like toxic organic compounds, there aresix possible mechanisms for a metal resistance

system:(1) exclusion by permeability barrier;

(2/3) intra- and extra-cellular sequestration;

(4) active efflux pumps;

(5) enzymatic reduction; and(6) reduction in the sensitivity of cellular targets to

metal ions


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Arsenic

Appears in Group V of the periodic table Semi metallic

Exists in both organic and inorganic form

Four oxidation states: -3, 0, +3, +5

The elemental form of arsenic is As (0)

Major inorganic forms are As+5 & As+3

Organic forms : Monomethylarsonic acid (MMA),Dimethylarsinic acid (DMA), and

Trimethylarsine oxide (TMAO)

In an aerobic environment, As (V) is dominant and is co-precipitate with orabsorb into iron oxyhydroxides under acidic and moderately reducingconditions.

Protonated forms are, H3As03, H2As03-

, and HAs032-

. Under extreme reducing conditions, elemental arsenic and arsine AsH3 may

be present.

Does not from complexes with anions such as Cl- and S042-.

Methylated form of arsine are, dimethyl arsine HAs (CH3)2 andtrimethylarsinic acid (CH3) As02H2 and dimethylarsenic acid (CH3)2As02H.


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Strucutre of arcenic compounds


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Toxicity

It replace phosphate in many biochemical

reactions

Presence of arsenic leads to arsenolysisand depletion of ATP.

Its Organic forms interact with specific

functional groups in enzymes, receptors,

or coenzymes.


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PhoEPhoE

PstABC ArsBPit

ArsA

ArsC

As(V)

As(V) As(III)

As(III)

OM

PR

CPM

CT

As(III)As(V)

arsR D A B C

Regulation :-

Transport of and resistance to As(V) in Escherichia coli


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Arsenic metabolism

As(V) enter through phosphatetransport systems such as thePst ABC ATPase.

As(V) is reduced to As(III) by

ArsC, a GrxGSH-linkedenzyme.

The resulting As(III) is pumpedout of the cells by the

ArsA/ArsB ATPase.

The arsRDABC operon isregulated by the ArsRrepressor protein and ArsDco-regulator protein.


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Transport and metabolism of arsenic in Bacillus subtilis

Three products of the

ars-operon have

known functions.(1)ArsC reducesAs (V) to As (III),

(2)ArsB activelypumps As (III) out of

the cell,while(3)ArsR regulates theexpression of the

operon.

As (V) enters the cell via the phosphate uptake system (Pit).As (III) is thought to be taken up by glycerol transporters (GlpF).


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The ars-operon contains four

genes: arsR, yqcK, arsB, and arsC.


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1) an oxyanion [As(V), phosphate, or sulfate] is bound noncovalently to theenzyme; a covalent thiarsahydroxy intermediate forms with the thiolate ofCys12.

2) a glutathionylated intermediate is formed

3) is then reduced to novel ArsC-S-As+-O intermediate

4) In the final step,this enzyme-As(III) complex dissociates to release freeAs(III).

The Reaction Mechanism OfArsC


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Reductases involved in As(V) detoxification have apparently evolved independentlyat least three times. One family is typified by the E. coliplasmid R773 ArsC thatuses Grx and GSH as reductants.

A second family represented by the S. aureus plasmid pI258 ArsC uses thioredoxin(Trx) as a reductant and is related to a family of lmwPTPases.

Acr2p is the only eukaryotic arsenate reductase so far identified. It belongs to thesuperfamily of PTPases that includes the Cdc25a cell-cycle phosphatase.

3 familiesof

Arsenate

Reductases


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CONCLUSION

All these mechanisms aim at reducing theintracellular concentration of a metal so as toprotect the cellular targets

Free metal ions as they are likely to be moretoxic than the bound ones

Adaptation to live in such conditions

Mechanisms of toxic ion resistance may help in

basic understanding of the physiology of the celland in enhancing the ability of microorganismsto extract deleterious ions from the environment


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DISCUSSION


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REFERENCE Agency for Toxic Substances and Disease Registry, ATSDR. 1999.

http://www.atsdr.cdc.gov. Adebowale Adeniji (2004). Bioremediation of Arsenic, Chromium, Lead, and

Mercury

Lesle Lai. (2003): The carcinogenicy, Mutagenicity, and Toxicity of Arsenic andCadmium, SS Biology, MIT

Bushra Muneer, (2005). Role of Microorganism in Remediation of Heavy Metal inthe Wastewater of Tanneries

Mohammad Iqbal Lone, Zhen-li He, Peter J. Stoffela, Xiao-e Yang, (2008).Phytoremediation of heavy metal polluted soils and water:Progresses andperspectives Lone et al. / J Zhejiang Univ Sci B 2008 9(3):210-220

Riina Turpeinen(2002). Interactions between metals, microbes and plants Bioremediation of arsenic and lead contaminated soils

Turpeinen, R., Pantsar-Kallio, M., Hggblom, M., Kairesalo, T. Influence ofmicrobes on mobilization, toxicity and biomethylation of arsenic in soil. TheScience of the Total Environment 236 (1999) 173-180.

CHAALAL Omar; ZEKRI Adulrazag Y.; ISLAM Rafiq. Uptake of heavy metals bymicroorganisms: An experimental approach 2005, vol. 27, no1-2, pp. 87-100 [14page(s) (article)] (25 ref.)

http://www.cob.lu.se/arsenic/proj_descr.html

http://www.bionet.schule.de/schulen/novaky/heavy_metals/en/hm01.htm


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Thank You

For any query please write to [email protected]

Heavy Metal Bhuvan

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