Table 5.

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

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