[基础科学]中科院生物化学课件

O H 2N CH CH 2 C O
-
O
C
OH
H 2N
CH CH 2
C
OH
O
CH 2 C OO
Aspartate (Asp, D )
G lu ta m a te (G lu , E )
天门冬氨酸
谷氨酸
Aspartate (Aspartic acid) and Glutamate (Glutamic acid) are only two amino acids with negatively charged (i.e. acidic) side chains. In each case the pKa of the side chain carboxyl group is about 4.4, so at neutral pH the equilibrium is way over in favour of the ionised form.
Alanine (Ala, A)
丙氨酸
Valine (Val, V)
缬氨酸
O H 2N CH CH 2 CH CH 3 CH 3 C OH
O H 2N CH CH CH 2 CH 3 C CH 3 OH
Leucine (Leu, L)
亮氨酸
Isoleucine (Ile, I)
异亮氨酸
The nonpolar amino acids are characterised by having only carbon and hydrogen in their side chains. They are generally unreactive, but the fact that they are hydrophobic gives them an important role to play in determining the 3-D structure of proteins, as they tend to cluster on the inside of the molecule. Alanine, Valine, Leucine and Isoleucine have saturated hydrocarbon R groups (i.e. they only have hydrogen and carbon linked by single covalent bonds).
Titration of the α-Carboxyl and α-Amino Groups of an Amino Acid.
Histidine, an essential amino acid.
At pH 6.5, Histidine can be uncharged or positively charged depending upon its local environment. The unprotonated imidazole is nucleophilic and can serve as a general base, while the protonated form can serve as a general acid. The ability to switch between these two states can give Histidine an important role in the catalytic mechanism of enzymes and explains why it is often found in the active site.
L-methionine may protect against the toxic effects of hepatotoxins, such as acetaminophen (对乙酰氨基酚, 又称扑热息痛).
脯氨酸
Proline is the only cyclic amino acid. Proling has no free amino group, is an imino acid (亚氨基酸).
¾Tryptophan is also structurally related to Alanine,
but with a two ring (bicyclic) indole (吲哚) group added in place of the single aromatic ring found in Phenylalanine. The presence of the nitrogen group makes Tryptophan a little less hydrophobic than Phenylalanine.
O H2N CH CH2 C OH
H 2N CH CH 2 O C OH
HN
Phenylalanine (Phe, F)
Trptophane (Trp, W)
苯丙氨酸
色氨酸
¾Phenylalanine is Alanine with an extra Phenyl
group on the end. Phenylalanine is highly hydrophobic and is found buried within globular proteins. Phenylalanine is an essential amino acid.
O
O H2N CH H C OH
O H 2N CH CH 2 OH Serine (Ser, S) C OH
H 2N
CH CH 2
C
OH
Glycine (Gly,G)
甘氨酸
OH
丝氨酸
Tyrosine (Tyr, Y)
酪氨酸
O
O
OH
H2N
CH
C
H2N
CH CH CH 3
C OH
OH
CH2
SH
Cysteine (Cys, C)
The pKa of an acid (or protonated base) is simply expressed as: pKa = - log (Ka)
pKa Values of Amino Acids
An amino acid such as glycine contains two ionizable groups: an α-carboxyl group and a protonated α-amino group. As base is added, these two groups are titrated .
2.1.2. Classification (分类)
(1) Hydrophobic(疏水的), aliphatic(脂肪族)amino acids－the side chains are poorly soluble in water (nonpolar)
O H2N CH CH3 C OH
O H 2N CH CH CH 3 C CH 3 OH
Threonine (Thr, T)
苏氨酸
半胱氨酸
Serine and Threonine in the context of proteins may be regulated by phosphorylation. They are also very important in the active sites of many enzymes.
Lysine (Lys, K )
Histidine (His, H)
精氨酸
赖氨酸
组氨酸
Three amino acids are not only polar but can also carry a positive charge and are therefore highly hydrophilic(亲水的). Lysine and Arginine both have pKa around 10.0 and are therefore always positively charged at neutral pH.
(3) Polar(极性), charged(带电荷)amino acids
O H 2N CH CH 2 CH 2 CH 2 NH C NH 2 Arginine (Arg, R ) NH 2 +
CH 2 O
O
OH
C
OH
H 2N
CH CH 2 CH 2
C
H 2N
CH CH 2
C
OH
HN N +H
CH 2 NH 3 +
There is no H-bond donor in the proline peptide bond.
Hale Waihona Puke The presence of proline leads to a bend or kink in the polypeptide chain.
(2) Hydrophobic (疏水性), aromatic (芳香族) amino acids
Cysteine (Cys) is not very polar, and is often classified as nonpolar. When Cys is exposed to air, it oxidizes to form Cystine, which is two cysteine molecules joined by a disulfide bond. Disulfide bond is important in the formation and maintenance of the folded structure in many proteins.
Chapter Two
Amino Acid and Protein