Download Lecture 5-Bioinorganic Chemistry

Bioinorganic Chemistry
Study of metal species in biological systems
•metal ion transport and storage,
•Metallohydrolase enzymes,
•metal-containing electron transfer proteins,
•oxygen transport and activation proteins,
•bioorganometallic systems such as hydrogenases and alkyltransferases,
•enzymes involved in nitrogen metabolism pathways.
Biological functions of selected metal ions
Metal
Function
Na, K
Charge carrier, osmotic balance
Mg, Zn
Structural, hydrolase, isomerase
Ca
Structural, charge carrier
V, Mo
Nitrogen fixation, oxidase
Mn
Photosynthesis, structural, oxidase
Fe, Cu
Dioxygen transport and storage,
electron transfer, oxidase
Ni
hydrogenase, hydrolase
Na Mg
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn
Y Zr Nb Mo Tc Ru Rh Pd Ag Cd
La Hf Ta W Re Os Ir Pt Au Hg
Naturally occurring in biology
Used as probes
Chemical elements essential to life forms can be divided into the following
(i) Bulk elements: C, H, N, O, P, S
(ii) Macrominerals and ions:
Na, K, Mg, Ca, Cl, PO43-, SO42-
(iii) Trace elements: Fe, Zn, Cu
(iv) Ultratrace elements comprises of
(a) non-metals: F, I, Se, Si, As, B
(b) metals: Mn, Mo, Co, Cr, V, Ni, Cd, Sn, Pb, Li
Essentiality of elements is defined by
(1) A physiological deficiency appears when the element is
removed from the diet
(2) The deficiency is relieved by the addition of that element
to the diet
(3) A specific biological function is associated with the element
Every essential element follows a dose-response curve
At lowest dosages organism does not survive
In deficiency regions, the organism exists with less than
optimal functions
After optimal dosage (plateau region), higher dosage cause
toxic effects in the organism eventually leading to lethality
Active Site and Enzyme Substrate Complex
The active site of an enzyme is the region that binds the substrate
and contributes the amino acid residues that directly participates
in the making and breaking of chemical bonds
Generalizations
1) Enzymes are usually very large compared to the substrate
Only a small portion is involved in ES complex
Rest portion is involved in control and maintaining of structure
2) The substrate is bound by relatively weak forces
ΔG E-S complex = (12 to 36) KJ mol-1
(strength of a covalent bond is upto ~ 450 KJ mol-1)
3) Active sites are designed to exclude H2O
Surrounded with non-polar amino acids to create a hydrophobic environment
Essential for substrate binding and product formation (Catalysis)
Specificity
Active site provides specificity for its particular substrate
Substrate has a matching shape to fit into the active site
(Lock and Key mechanism)
Formation of Enzyme-Substrate Complex is thus crucial to the
product formation
Evidence for Enzyme-Substrate Complex
(1) At constant [E], increasing the [S] will increase the reaction rate until a
maximum velocity is reached,
(2) Isolation of E-S complex
(3) X-ray diffraction studies of E-S complex
(4) Spectroscopic studies of E-S complex
Active sites of Enzymes
His(N)
Zn
Glu(O)
OH 2
peptide hydrolysis
(removes terminal amino
acids f rom proteins)
His(N)
Carboxy peptidase
His(N)
His (N)
Zn
OH2
H 2O + CO
H 2CO 3
H + + HCO 3-
His(N)
Carboxy anhydrase
NAD+
Cys(S)
Cys(S)
His(N)
Zn
NADH
OH2
CH 3CH 2OH
Liver Alcohol dehydrogenase
CH 3CHO
Active-Sites of Enzymes
O
Hemoglobin
O
CO 2-
CO 2N
N
II
Fe
N
CO 2-
O2
N
N
CO 2-
II
Fe
N
N
N
Hemocyanin
NH
HN
NH
N
H
N
N
N
N
HN
N
H
CuII
O
N
N
N
NH
NH
O
CuII
N
N
N
N
O2
CuI
CuI
Nh
N
N
H
N
H
N
H
Hemerythyrin
H
N
H
N
H
H
O
N
HN
O
O
Fe
O
HN
N
H
O
O
FeII
N
N
N
O
HN
O2
II
Fe
N
N
HN
N
II
N
NH
O
O
O
N
FeIII
N
O
O
N
H
NH
Porphyrins
Porphyrins are tetrapyrrole macrocycles with conjugated double
bonds and various groups attached to the perimeter
R
R
N
R
R
HN
NH
R
R
N
R
R
variation of substituents facilitates the tuning of electron-donating and
electron-withdrawing ability of the ligand
The porphyrins can accept two hydrogen ions to form+2 diacids or donate
two protons to form -2 dianions
Porphyrins are found in many metalloenzyme
Enzyme
Function
Fe-porphyrin
Cytochrome
electron transfer
Fe-porphyrin
Hemoglobin
Myoglobin
dioxygen carrier
Mg-porphyrin
Chlorophyll
photosynthesis
Cytochromes
Cytochromes are electron transfer proteins
There are three types of cytochromes depending upon the porphyrin types
cytochrome a, cytochrome b, cytochrome c
s-cys protein
s-cys protein
HO
HO
N
N
N
N
N
N
Fe
Fe
Fe
N
N
N
N
N
N
O
H
HO
OH
O
O
a
HO
OH
O
O
b
HO
OH
O
O
c
The prosthetic group in all cytochromes comprises of four heme units
They have a molecular weight of about 12,400
Active site differences between Hemoglobin and Cytochrome
O
S (cys)
O
N
N
N
N
N
Fe
Fe
N
N
N
N
(His)
N
(His)
Hemoglobin
Cytochromes
Depending upon the ligand, the redox potential of a given cytochrome
can be tailored to meet specific need in electron transfer schemes
(photosynthetic versus respiration)
The potentials are such that the electron flow is from
b
c
a
O2
Cytochrome a is capable of binding O2 and reducing them
Cytochrome a is responsible for severe toxicity of CNCN- binds to the 6th site and stabilize FeIII to such an extent that it
cannot participate in electron transfer schuttle
Hemoglobin
O
O
N
N
N
N
Fe II
N
N
Fe II
N
N
N
(His)
N
(His)
High Spin
paramagnetic
t2g4 eg2
Deoxyhemoglobin
Low Spin
diamagnetic
t2g6 eg0
Oxyhemoglobin
Deoxyhemoglobin is the form of hemoglobin without the bound oxygen.
The oxyhemoglobin has significantly lower absorption (660 nm) than
deoxyhemoglobin (940 nm). This difference is used for measurement of
the amount of oxygen in patient's blood by pulse oximeter.
The size of Fe2+ increase by 28% on going from
Low spin (oxyhemoglobin) (0.61 Å)
to
High spin (Deoxyhemoglobin) (0.78 Å)
The Fe2+ in deoxyhemoglobin is too large to fit in the ring and is
situated (0.7-0.8)Ao above the ring
Thus, presence of O2 changes the electronic arrangement of Fe2+ and
distorts the shape of the complex
The globular protein prevents the irreversible oxidation of Fe(II) to Fe(III)
Cooperativity
When O2 binds to
one sub-unit
Fe2+ contracts, moves into plane
of porphyrin ring
triggers conformational
changes in the globin chain
moves the histidine attached to it
translated through
H-bond network
Enhances the ability of other
three units to bind O2
This phenomenon is
called cooperative effect
In a similar way when the blood reaches the muscle, only one O2 is released,
the others are released even more easily due to the cooperative effect in reverse
picket fence porphyrin