Chemistry Reference
In-Depth Information
Chapter 10
Magnesium
e
Phosphate Metabolism
and Photoreceptors
Introduction
197
Magnesium-Dependent Enzymes
198
Phosphoryl Group Transfer Kinases
199
Phosphoryl Group Transfer
e
Phosphatases
203
Stabilisation of Enolate Anions
e
The Enolase Superfamily
204
Enzymes of Nucleic Acid Metabolism
205
Magnesium and Photoreception
210
INTRODUCTION
Mg
2
þ
is one of the most abundant elements in the earth's crust and in the human body, and the most abundant
divalent cation within cells. Around 50% of total Mg
2
þ
resides in bone, the remainder essentially within cells;
50% of cytosolic Mg
2
þ
is bound to ATP, and most of the rest, together with K
þ
, is bound to ribosomes. The
intracellular concentration of free Mg
2
þ
is around 0.5 mM. Less than 0.5% of the total body Mg
2
þ
is in plasma,
where its concentration is maintained within fairly strict limits.
Mg
2
þ
has properties which make it quite unique among biological cations. Inspection of
Table 10.1
reveals
that of the four common biological cations, the ionic radius of Mg
2
þ
is much smaller than the others, whereas its
TABLE 10.1
Properties of Common Biological Cations
Ionic
Radius (
˚
)
Hydrated
Radius (
˚
)
Ionic
Volume (
˚
3
)
Hydrated
Volume (
˚
3
)
Exchange
Rate (s
1
)
Transport
Number
Cation
Na
þ
8 10
8
0.95
2.75
3.6
88.3
7
e
13
K
þ
10
9
1.38
2.32
11.0
52.5
4
e
6
Mg
2þ
10
5
0.65
4.76
1.2
453
12
e
14
Ca
2þ
3 10
8
0.99
2.95
4.1
108
8
e
12
(from
Maguire & Cowan, 2002
)
.
hydrated radius is the largest of all four. This means that the volume of the hydrated Mg
2
þ
cation is 400 times
larger than its ionic volume (since the radius enters into the equation to the third power), compared to values
around 25 times for Na
þ
and Ca
2
þ
, and a mere 5 times for K
þ
.
