Heme is essential
for the survival of virtually all living systems—from
bacteria, fungi, and yeast, through plants to animals. No eukaryote
has been identified that can survive without heme. There are thousands
of different proteins that require heme in order to function properly,
and these are responsible for processes such as oxygen transport,
electron transfer, oxidative stress response, respiration, and catalysis.
Further to this, in the past few years, heme has been shown to have
an important regulatory role in cells, in processes such as transcription,
regulation of the circadian clock, and the gating of ion channels.
To act in a regulatory capacity, heme needs to move from its place
of synthesis (in mitochondria) to other locations in cells. But while
there is detailed information on how the heme lifecycle begins (heme
synthesis), and how it ends (heme degradation), what happens in between
is largely a mystery. Here we summarize recent information on the
quantification of heme in cells, and we present a discussion of a
mechanistic framework that could meet the logistical challenge of
heme distribution.