Passage
Tumor hypoxia is a marker of resistance to chemotherapy and radiation. The low oxygen saturation in solid tumors activates hypoxia-inducible factors (HIFs), which stimulate the expression of genes required for angiogenesis, erythropoiesis, and glucose utilization. Under normal conditions, HIF-DNA binding is interrupted by HIF-prolyl hydroxylases (HIF-PHDs), enzymes that hydroxylate aliphatic residues such as leucine or proline on HIFs. These hydroxylated residues serve as markers for the ubiquitin-proteasome system. Researchers have proposed that changes in oxygen-dependent pathways such as the citric acid cycle can regulate HIF-PHD activity.Experiment 1A kinematic study was conducted to test the effect of citric acid cycle intermediates on HIF-PHDs. Fumarate and succinate were examined with purified HIF-PHD against increasing concentrations of 2-oxoglutarate (2-OG), an HIF analog.

Figure 1 Activity of HIF-PHD exposed to succinate and fumarate in vitro based on 2-OG catalysisExperiment 2Researchers measured HIF-1a, a HIF subunit, using western blot analysis in wild-type (FH⁺) and fumarate-deficient (FH⁻) cells that were treated with fumarate under normoxic and hypoxic conditions (Figure 2).

Figure 2 HIF-1a in FH⁺ and FH⁻ cells grown in vitro under normoxic (N) and hypoxic (H) conditions
Adapted from Koivunen P, Hirsilä M, Remes AM, Hassinen IE, Kivirikko KI, Myllyharju J. Inhibition of hypoxia-inducible factor (HIF) hydroxylases by citric acid cycle intermediates: possible links between cell metabolism and stabilization of HIF. J Biol Chem. 2007;282(7):4524-32.
-The peptide substrate (PS) binds HIF-PHDs outside of the active site. PS binds to both HIF-PHD and HIF-PHD-2-OG complexes with equal affinity. According to the passage, which graph would represent the effect of PS on HIF-PHD activity?
A)

Polyglutaraldehyde (PGA) Can Form Linkages with Amines

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