across a wide range、existence 怎么理解？

Hypoxia-inducible factor (HIF).20 This is a ubiquitous enzyme which, under hypoxic conditions, initiates transcription of many different genes to help the cell survive hypoxia (page 276). HIF comprises two subunits, an a-subunit which is oxygen sensitive and a constitutive b-subunit. In normoxic cells the a-subunit of active HIF has a half-life of only 5 min because of its rapid inactivation by hydroxylation; this reaction is dependent on prolyl hydroxylase-domain (PHD) enzymes. PHD activity is dependent on Po2 across a wide range of values, and the effects of HIF on transcription is also oxygen-dependent,6 so both its existence and activity are oxygen-sensitive

缺氧诱导因子（hypoxia-inducible factor，HIF）²⁰。HIF是一种普遍存在的酶，在缺氧时能启动众多不同基因的转录，帮助细胞存活（第 276 页）。HIF包含两个亚基，一个氧敏感的α-亚基和一个构成性β-亚基。氧正常的细胞中，具有生物学活性的HIF的α-亚基被脯氨酰羟化酶（prolyl hydroxylase-domain，PHD）羟基化迅速失活，其半衰期仅为5分钟。在Po2值很大范围变化时，PHD 活性都会随Po2值的变化而变化，而HIF对转录的影响也是氧依赖性的，而HIF对转录的影响也是氧依赖性的⁶，所以HIF的数量和活性都是氧敏感的。

across a wide range、existence 怎么理解？