| Claim/Topic | Key details | Evidence type | Source (authors, year, journal) | URL | Notes/quantitative values |
|---|---|---|---|---|---|
| Core function | **HdeA (UniProt P0AES9)** is the **periplasmic acid-stress chaperone** of *E. coli* K-12; it is a small (~11 kDa) ATP-independent holdase that prevents acid-denatured **periplasmic proteins** from aggregating and assists refolding after pH neutralization. It is inactive as a folded dimer at neutral pH and active in acid as a partially unfolded monomer/disordered state. (pqac-00000000, pqac-00000004, pqac-00000008) | Review + primary | Kim et al., 2021, *Front. Mol. Biosci.*; Yu et al., 2017, *Biochemistry*; Li et al., 2024, *Microorganisms* | https://doi.org/10.3389/fmolb.2021.678697; https://doi.org/10.1021/acs.biochem.7b00724; https://doi.org/10.3390/microorganisms12091774 | Functional window mainly **pH 1–3** for HdeA; ATP-independent anti-aggregation chaperone. |
| Activation mechanism and pH thresholds | Acidification protonates acidic residues, destabilizing electrostatic contacts and the dimer interface; HdeA undergoes **dimer-to-monomer transition** plus **partial unfolding/order-to-disorder conversion**, exposing hydrophobic client-binding patches. In substrate-free conditions, major activation occurs around **pH ~3 to 2**; HdeA is largely inactive above ~pH 3–4 and strongly active below ~pH 3–3.5. Mild acid can transiently stabilize the dimer near **pH ~5**, with a sharp folded-dimer to unfolded-monomer transition between **pH 3 and 2**. (pqac-00000001, pqac-00000003, pqac-00000005, pqac-00000006, pqac-00000007) | Primary structural/biophysical | Yu et al., 2017, *Biochemistry*; Garrison & Crowhurst, 2014, *Protein Sci.*; Dahl et al., 2015, *JBC*; Salmon et al., 2018, *J. Mol. Biol.*; Wu et al., 2008, *Biochem. J.* | https://doi.org/10.1021/acs.biochem.7b00724; https://doi.org/10.1002/pro.2402; https://doi.org/10.1074/jbc.m114.612986; https://doi.org/10.1016/j.jmb.2017.11.002; https://doi.org/10.1042/bj20071682 | HdeA active mainly **pH 1–3**; HdeB activates earlier (**~pH 4.5**) and HdeA later (**≤3.5**) in comparative studies. |
| Localization | HdeA acts in the **periplasm**, the compartment that rapidly equilibrates with external acidity and is therefore vulnerable to acid-induced protein unfolding/aggregation. (pqac-00000000, pqac-00000004) | Review + primary | Kim et al., 2021, *Front. Mol. Biosci.*; Yu et al., 2017, *Biochemistry* | https://doi.org/10.3389/fmolb.2021.678697; https://doi.org/10.1021/acs.biochem.7b00724 | Matches UniProt precursor/periplasmic annotation for P0AES9. |
| Client proteins | Experimentally discussed native/periplasmic clients include **SurA, MalE, OppA** by NMR interaction studies; comparative proteomics identified common or preferred clients including **SurA, BglX, DegP, DsbA, OppA**, plus proteostasis factors such as **DsbC, DsbG, PpiD, DegQ, Tsp, PtrA**. **DppA** was HdeA-preferred in the 2016 proteomics study. (pqac-00000001, pqac-00000018) | Primary NMR + proteomics | Yu et al., 2017, *Biochemistry*; Zhang et al., 2016, *PNAS* | https://doi.org/10.1021/acs.biochem.7b00724; https://doi.org/10.1073/pnas.1606360113 | Broad client scope focused on **acid-unfolding periplasmic proteins**; ~**80%** of identified clients were common to HdeA and HdeB. |
| Stoichiometry / binding mode | HdeA binds substrates as a **heterogeneous, amphiphilic, dynamic complex** rather than a single rigid stoichiometric complex. Reported binding plateau reached roughly **10 HdeA molecules per substrate** for **OppA** and **MalE** under assay conditions. Termini help maintain complex solubility; deletion mutants can still bind but show reduced solubility/co-precipitation. (pqac-00000017, pqac-00000004) | Primary NMR/mechanistic | Yu et al., 2017, *Biochemistry* | https://doi.org/10.1021/acs.biochem.7b00724 | Approximate plateau stoichiometry **~10:1 (HdeA:substrate)**, likely reflecting in vitro excess rather than physiological fixed stoichiometry. |
| Periplasmic chloride / Donnan effect | Extreme acid stress in the periplasm is worsened by a **Donnan-effect chloride surge**, reported to exceed **0.6 M Cl-**, which accelerates protein aggregation and helps explain the need for HdeA/HdeB periplasmic chaperones. (pqac-00000000, pqac-00000016) | Review | Kim et al., 2021, *Front. Mol. Biosci.* | https://doi.org/10.3389/fmolb.2021.678697 | Useful physiological context for why HdeA is highly expressed and acid-essential. |
| Genetic phenotype / acid resistance role | **hdeA mutants show reduced survival/viability after low-pH exposure**; loss of HdeA function gives a strongly acid-sensitive phenotype. HdeA is a major chaperone at **pH ~2**, while HdeB contributes more at **pH 3**; both contribute to optimal acid survival in vivo. (pqac-00000002, pqac-00000007, pqac-00000013, pqac-00000014, pqac-00000016) | Primary genetics/biochemistry + review | Kern et al., 2007, *J. Bacteriol.*; Wu et al., 2008, *Biochem. J.*; Kim et al., 2021, *Front. Mol. Biosci.* | https://doi.org/10.1128/jb.01522-06; https://doi.org/10.1042/bj20071682; https://doi.org/10.3389/fmolb.2021.678697 | HdeA is especially important under **stomach-like pH 1–3**; complementation with both HdeA/HdeB gave better restoration than either alone in comparative studies. |
| Operon / regulation | **hdeAB** forms an operon on the **acid fitness island**. Expression is regulated by acid-response pathways including **EvgSA→YdeO** and is increased via **Crl through RpoS**; HdeA is reported as the **6th most abundant stationary-phase protein**. (pqac-00000000, pqac-00000016) | Review/regulatory synthesis | Kim et al., 2021, *Front. Mol. Biosci.* | https://doi.org/10.3389/fmolb.2021.678697 | Strong stationary-phase abundance underscores central role in acid preparedness. |
| 2016 proteomics pH windows | In vivo photocrosslinking/proteomics defined distinct client-binding windows: **HdeB begins binding at pH ≤4.5**, whereas **HdeA begins at pH ≤3.5**. Aggregation assays at **pH 2** showed HdeA more effective than HdeB for some clients; both improved soluble **SurA** at pH 2. (pqac-00000011, pqac-00000018) | Primary proteomics/aggregation assays | Zhang et al., 2016, *PNAS* | https://doi.org/10.1073/pnas.1606360113 | Assay details reported include **SurA:chaperone 1:1** at pH 2 and in vivo crosslinking after **pH 2.3 for 30 min** plus **365 nm UV for 15 min**. |
| Quantitative thermodynamics / structural switch | HdeA self-association shows nonmonotonic pH dependence, with maximum dimer stability near **pH ~5**; enthalpy for dimer→monomer dissociation at **pH 2.3** was reported as **10.6 ± 0.3 kcal/mol**. Protonation of **Glu37** contributes to activation, and a low-populated partially folded intermediate may participate in unfolding/function. (pqac-00000006) | Primary biophysics | Salmon et al., 2018, *J. Mol. Biol.* | https://doi.org/10.1016/j.jmb.2017.11.002 | Supports updated mechanistic view that activation is multistep, not a simple binary switch. |
| 2024 acid-stress review & engineering stats | Recent review reiterates HdeA as the **extreme-acid** periplasmic chaperone (**pH 1–3**) and highlights engineering of acid resistance for industrial strains. Quantitative engineering examples summarized in the review include **336.3-fold** survival increase and **113.6%** increase in D-lactic acid titer via **HypB/HypC** engineering, and **4509.6-fold** survival increase at **pH 4.0** via **rffG** overexpression; these are acid-resistance context metrics rather than HdeA-specific interventions. (pqac-00000008, pqac-00000009) | 2024 review / application synthesis | Li et al., 2024, *Microorganisms* | https://doi.org/10.3390/microorganisms12091774 | Useful for applied context: HdeA/HdeB are part of the acid-resistance toolkit leveraged in strain design, though the cited quantitative gains here are not direct hdeA overexpression data. |
| 2024 StressME mention | **StressME** integrates the prior **AcidifyME** acid-stress framework and explicitly includes **periplasmic chaperone protection mechanisms**, noting that **HdeA/HdeB** are major contributors to acid-response proteome allocation in *E. coli*. (pqac-00000012) | 2024 computational model | Zhao et al., 2024, *PLOS Comput. Biol.* | https://doi.org/10.1371/journal.pcbi.1011865 | Provides systems-level support that periplasmic chaperones are major acid-stress investment classes, though no HdeA-only quantitative coefficient is given in the excerpt. |
| 2024 engineered strain SC3124 metrics | In an engineered acid-tolerant strain (**SC3124**), a synthetic module containing **gadE + hdeB + sodB + katE** improved mild-acid performance. Final **OD600 at pH 6.0** was **131%** and **124%** of parent MG1655 measured at pH 6.8 and pH 6.0, respectively. When transferred to a lysine-production background, the module increased lysine yield to **115% (pH 6.0)** and **118% (pH 6.8)** versus parent strain in **1.3-L bioreactors**. (pqac-00000010, pqac-00000015) | 2024 engineering / transcriptomics | Qin et al., 2024, *Microorganisms* | https://doi.org/10.3390/microorganisms12081565 | Directly demonstrates modern exploitation of periplasmic acid-chaperone logic in strain engineering; uses **HdeB** rather than HdeA because the target regime was **mild acid (pH 5–6 / tested at pH 6.0)**. |
| Current annotation summary | Functional annotation for **P0AES9 / hdeA**: **acid-activated periplasmic holdase chaperone**, member of HdeA family, precursor exported to periplasm; protects acid-labile periplasmic proteins during extreme acid stress, then releases clients on neutralization for refolding. (pqac-00000000, pqac-00000004, pqac-00000008) | Integrated from review + primary | Kim et al., 2021; Yu et al., 2017; Li et al., 2024 | https://doi.org/10.3389/fmolb.2021.678697; https://doi.org/10.1021/acs.biochem.7b00724; https://doi.org/10.3390/microorganisms12091774 | Best-supported primary role is **protein quality control in the acidic periplasm**, not catalysis or transport. |


*Table: This table condenses the core functional annotation of E. coli HdeA (UniProt P0AES9), including mechanism, localization, regulation, client proteins, and quantitative findings. It also highlights recent 2024 systems and engineering context relevant to acid-stress biology.*