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thus participates – as a killer agent for infectious germs – in the functioning of the immune
system; (3) eNOS, in the endothelial tissue cells, where it controls the tonicity of the vascular
muscles and thus the blood pressure. The NO-induced relaxation of the vascular muscles is
also the basis for the medication of hypertension and angina pectoris with compounds which
set free NO under physiological conditions, such as amylnitrite, (C 5H 11NO 2); nitroglycerin
(glyceroltrinitrate CH 2(ONO 2)-CH(ONO 2)-CH 2(ONO 2)) and nitroprussid sodium (disodium
pentacyanido-nitrosylferrate Na 2[Fe(NO)(CN) 5]).

NO is also used by the glow worm (lightning bug, firefly) to switch on its glow organs.
This luminescence can be traced back to the oxidation, by O 2, of luciferyl-AMP (AMP =
adenosine-monophosphate) via peroxoluciferin to oxoluciferin; cf. Fig. 24. To start this oxygen
consuming formation of peroxoluciferin, the glow worm triggers NO synthesis. The NO is
used to block mitochondrial cytochromes (by coordination of NO to Fe) and thus the
consumption of oxygen in respiration. The O 2 thus becomes available for triggering
luminescence. Other organisms capable of bioluminescence also employ this mechanism. An
example is Nocticula scintillans, a dinoflagellate responsible for marine phosphorescence.
.
NO
NO
COOH
HO S N
L u c i f e r i n

N S

ATP(Mg 2+ )
(Luciferase) O O O
HO P P O
O OH
Luciferyl-AMP
O 2 O
2 O
OOH HO S N
HO S N O
C HOAMP N S
AMP + CO 2 + Licht
N S + CO + light Oxiluceferin
2
Peroxoluciferyl-AMP

Figure 24. NO induced luminescence in the glow worm.

Larger amounts of NO are toxic, because NO binds to the iron and copper centres of
enzymes depending on these metals. Haemoglobin binds NO ten-thousand fold more effective
-
than O 2. On coordination, NO is reduced to NO , which is isoelectronic with O 2 and binds, as
O 2, in the bent end-on mode. Additional NO toxicity arises from the fact that NO nitrosylates
amines, via the intermediate formation of nitrous acid, to form carcinogenic nitrosamines:
-
+
NO + H 2O → HNO 2 + e + H
R 2NH + HNO 2 → R 2N-NO + H 2O
(Nitrous acid is also formed, under physiological conditions, by reduction of nitrate, present in
e.g. leafy vegetables.)

9. The biochemistry of zinc

2.5 g of zinc per 70 kg body weight makes Zn the second-to-most abundant transition
metal of biological importance. Contrasting iron, copper, manganese and molybdenum, zinc is
2+
10
not redox active (valence electron configuration d ). In zinc proteins, Zn takes over either a