Biology Reference
In-Depth Information
Table 4.1.
Classification of human proteins by 'invention period' (after Doolittle, 1992)
Ancient proteins
First editions
: direct descent to both human and contemporary prokaryotes. Mostly
mainstream metabolic enzymes (e.g. triosephosphate isomerase).
Second editions
: homologous sequences in humans and prokaryotes but apparently serve
different functions (e.g. 27% identity between glutathione reductase
(human red blood cells) and mercury reductase
(
Pseudomonas
).
Middle age proteins
Proteins found in most eukaryotes but prokaryotic counterparts are as yet unknown (e.g. actin).
Modern proteins
Recent vintage
: proteins found in animals or plants but not both. Not found in prokaryotes
(e.g. collagen).
Very recent inventions
: Proteins confined to vertebrates (e.g. albumin).
Recent mosaics
: Modern proteins resulting from exon shuffling (e.g. low density lipoprotein
receptor).
Another example of human specificity is provided by the salivary amylase
(
AMY1
) genes clustered on human chromosome 1p21: three such genes are to be
found in the human genome whereas only one is present in chimpanzee
(Samuelson
et al
., 1990). The most parsimonious explanation is that amplification
of the
AMY1
gene sequences has occurred during the last 7 Myrs, the time since
human and chimpanzee last shared a common ancestor. Similarly, the high-affin-
ity immunoglobulin receptor genes (
FCGR1A
, 1q21;
FCGR1B
, 1p12;
FCGR1C
,
1q21) were triplicated from a single primate ancestral
Fcgr1
gene about 3 Myrs
ago, after the divergence of chimpanzees from the human lineage (Maresco
et al
.,
1996). Finally, evidence for substantial deletions/translocations in the chim-
panzee genome as compared to human is apparent from pulsed field gel elec-
trophoretic studies of the region between the
HLA-B
and
TNF
genes on
chimpanzee chromosome 5p21.3 (Leelayuwat
et al
., 1993).
The gene coding regions of humans and chimpanzees are typically of the order
of 99% homologous (e.g.
BRCA1
; Hacia
et al
., 1998). As yet, however, there are no
known differences between the human and chimpanzee genomes in terms of
either the presence/absence of genes or gene copy number that could account for
the considerable anatomical and behavioral differences between the two species.
We may surmise that subtle differences in the expression of regulatory genes or
alternatively differences in the responsiveness of those genes which serve as tar-
gets for the action of regulatory proteins, may help to explain why humans are not
chimpanzees. Candidate genes could include not only those involved in develop-
mental regulation but possibly also those situated adjacent to human-specific
karyotypic rearrangements.
4.1.2 Human genes which originated after the divergence of Old World
monkeys and New World monkeys
The haptoglobin gene appears to have become amplified after the separation of
Old World monkeys from New World monkeys. The gorilla, chimpanzee,
