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Table 5 Summary of key enzymes and proteins involved in PHA metabolism and regulation identified from halophiles

From: Current developments on polyhydroxyalkanoates synthesis by using halophiles as a promising cell factory

Process

Proteins

Organism

Features

Refs.

PHA synthesis

PhaEC

Haloarchaea

Haloarcula marismortui

Class III type; constitutes the active PHA synthase; co-transcription; constitutive expression; conserved lipase box-like sequence, amino acid triad (Cys-Asp-His), conserved motif of class III PHA and longer C-terminal sequence

[104]

Haloarcula hispanica

[104]

Haloferax mediterranei

[41]

Halogranum amylolyticum TNN58

Conserved lipase box-like sequence and catalytic triad residues; 64% and 62% identity with PhaE and PhaC from Hfx. mediterranei

[46]

PhaC1

Haloferax mediterranei

Lipase box-like sequence, conserved motif of class III PHA synthase and longer C-terminal sequence

[105]

PhaC2

Ala instead of the last Gly in lipase box-like sequence; conserved motif not strongly conserved; longer C-terminal sequence missing, without PhaC function

[105]

PhaC3

Lipase box-like sequence, conserved motif of class III PHA synthase and longer C-terminal sequence

[105]

PhaC

Halorubrum lacusprofundi

High abundance at low temperature; C-terminal has 47% identity with the C-terminal of Haloferax mediterranei PhaC

[38]

PhaC

Halophilic bacteria

Halomonas elongata DSM2581

Two candidate genes; phaC1 is functional; unique serine instead of the first glycine in lipase box-like sequence; PhaC1 has affinity towards both 3HB and 3HV monomers

[101]

Halomonas sp. O-1

Halomonas sp. R5-57

Three candidate encoding genes; the third PhaC is truncated

[106]

Yangia sp. CCB-MM3

Class I type; two candidate encoding genes

[107]

PhaC1

Halomonas bluephagenesis TD01

Conserved catalytic triad (Cys-Asp-His) and the conserved lipase box-like; Ser instead of first Gly in lipase box-like sequence

[52]

PhaC2

Conserved catalytic triad (Cys-Asp-His) and the conserved lipase box-like; longer C-terminus; shorter N-terminus; Ala instead of the last Gly in lipase box-like sequence

BktB

Haloarchaea

Haloferax mediterranei

Two subunits, α and β; α is the catalytic subunit and the catalytic residues are Ser-His-His; β subunit comprises of oligo-sachharide binding domain

[109]

PhaA

PhaA

Halophilic bacteria

Halomonas elongata BK-AG18

One subunit, catalytic residues are Cys-His-Cys

[112]

PhaB

Haloarchaea

Haloferax mediterranei

NADPH-dependent; two candidate encoding genes, phaB1 and phaB2; PhaB1 and PhaB2 responsible for 3HB-CoA and 3HV-CoA formation

[113]

Haloarcula hispanica

NADPH-dependent; only FabG1 responsible for PHA synthesis

[114]

Halophilic bacteria

Halomonas bluephagenesis TD01

NADH-dependent

[115]

PHA regulation

PhaP

Haloarchaea

Haloferax mediterranei

Consists of conserved amino acids and aspartate/glutamate rich regions in C-terminal; lysine acetylated

[117, 119]

Halophilic bacteria

Halomonas bluephagenesis TD01

Three candidate encoding genes; only PhaP1 responsible for the amount and size of PHA granules

[116]

PhaR

Haloarchaea

Haloferax mediterranei

Consists of AbrB (antibiotic resistance protein B)—like domain; regulates function of PhaP

[118]

Halophilic bacteria

Halomonas bluephagenesis TD01

Regulates PHA synthesis; amphiphilic property; strong and robust emulsifier

[98, 116]

PHA degradation

PhaZh1

Haloarchaea

Haloferax mediterranei

Palatin-like protein; Contains classical lipase box-like

[121]

BdhA

Encoding gene located upstream of phaZh1; hydrolyses 3HB monomers generated by PhaZ1 from natural PHA granules

[121]

PhaJ

Dehydrates 3-hydroxyacyl-CoA to enoyl-CoA

[100]

PhaZ1

Halophilic bacteria

Halomonas bluephagenesis TD01

Lacks signal peptide, intracellular depolymerase

[52]

PhaZ2

PhaZ3

Signal peptide present, extracellular depolymerase