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Zinc group elements
Periodic Table
bulk elements
for some species
trace elements
Fe, Cu, Mo:
Electron-transfer
Redox proteins and enzymes
Oxygen carrying proteins
Nitrogen fixation
Zn:
Metalloenzymes
Structure promoters
Lewis acid
Not a redox catalyst!
Other metal ions: less well defined and more
obscure roles
Terrestrial distribution:
Zn Cd Hg (relatively low conc.)
Distribution in vivo:
Zn
In human being: ~ 2 g
Zn2+
• ~ 25-30 % in skin and bones
• ~ 50 % in the blood
~ 12-20 % plasma
~ 75-80 % erythrocytes
~ 3 % leucocytes
• Pancreas (b-cells)
• eye
Role of Zn2+ :
deficiency:
• disturbances of repr. system
• dwarfism
• skin lesions
• skeletal abnormalities
Proteins associated with Zn:
1. Helical:
insulin, phospholipase
2. b-sheet: carbonic anhydrase
superoxide dismutase
3. Mixed:
carboxypeptidase
alcohol dehydrogenase
4. Random: metallothioneins: Zn2+, Cd2+, Hg2+
(Cys – X – Cys)7
x=nonaromatic amino acid
Zn – metalloenzymes: 80!
Zn activated enzymes: 20!
(H2O)(1-2)
N
S
(H2O)(1-2)
Zn
O
Zn
N
C
O
S(N)
S(N)
Zn
N
S
S
S
Function of Zn in metalloenzymes
1. Structure-promoter
2. Substrate binder
3. Lewis acid
ad. 1.
ad. 2.
reactions catalyzed by
Zn- metalloenzymes:
hydrolysis of
– esters
– amides
– peptides
e.g.
OH
R
C
O
R’ + OH-
R
ad 3. Zn2+ (H2O)
O
O(-)
O
Zn2+
C
Zn2+
Zn2+(OH-) + H+
R’
H
His
His
O
O(-)
His
Zn2+
C
O
H
His
His
His
O = C = O + OH-
O
O
Zn2+
C
O(-)
HCO3-
Cd2+ (toxic):
Interferes with:
Zn2+ , Fe2+ metabolism
Cumulates in:
kidney
liver
seminiferous tubules
(kills the spermatozoa)
Hg
– Hg(l)
non toxic!
– Hg(g)
TOXIC
– Hg2+(aq)
TOXIC
Alkyl and aryl mercury toxic
Al3+
– Low availability (insoluble ~ pH 7)
– Ox. State: +3 (no redox reaction)
– Alzheimer’s disease: presenile dementia
Alumino-silicate plaques
Ga3+, In3+: moderately toxic
TI3+, TI+: extremely toxic
Pb2+, Pb4+: distinctly toxic
Cumulative effect!
Intake: ~ 300 mg/day
 Pipe: Pb (HCO3)2
 Plants accumulate
 Lead pollution: exhaust emission Pb(C2H5)4
Inhibits the biosynthesis of hem!