Essential Cell Biology 5th edition

67
C–O COMPOUNDS
Many biological compounds contain a carbon covalently
bonded to an oxygen. For example,
alcohol
aldehyde
ketone
carboxylic acid
esters
C
C
O
OH
C
C
H
C
H
C
C
C
O
H
O
OH
O
OH
HO C
C C
The –OH is called a
hydroxyl group.
The C O is called a
carbonyl group.
Esters are formed by combining an
acid and an alcohol.
acid alcohol ester
The –COOH is called a
carboxyl group. In water,
this loses an H + ion to
become –COO _ .
O
O
C
H 2 O
C–N COMPOUNDS
Amines and amides are two important examples of
compounds containing a carbon linked to a nitrogen.
Amines in water combine with an H + ion to become
positively charged.
C
N
H
H
H
+
H + C N H
Amides are formed by combining an acid and an
amine. Unlike amines, amides are uncharged in water.
An example is the peptide bond that joins amino acids
in a protein.
O
O
acid
C
OH
H 2 N
C
Nitrogen also occurs in several ring compounds, including
important constituents of nucleic acids: purines and
pyrimidines.
NH 2
O
amine
N
C
N
H
C
C
H
C
N
H
H
C
cytosine (a pyrimidine)
H
H 2 O
amide
SULFHYDRYL GROUP
The C SH is called a sulfhydryl group. In the amino acid cysteine, the sulfhydryl group may exist in
the reduced form, C SH or more rarely in an oxidized, cross-bridging form, C S S C
PHOSPHATES
Inorganic phosphate is a stable ion formed from
phosphoric acid, H 3 PO 4 . It is also written as P i .
Phosphate esters can form between a phosphate and a free hydroxyl group.
Phosphate groups are often covalently attached to proteins in this way.
HO
O
P
O _
O _
O
C OH P O _
HO
O
C O P O _
H 2 O
also
written as
O _ O _
C O
P
The combination of a phosphate and a carboxyl group, or two or more phosphate groups, produces an acid anhydride.
Because compounds of this type release a large amount of free energy when the bond is broken by hydrolysis in the cell,
they are often said to contain a “high-energy” bond.
H 2 O
O
O
O
C
P O _
also written as
HO
C O
“high-energy” acyl phosphate
O
bond (carboxylic–phosphoric
OH
O _
O P O _
acid anhydride) found in
C
H 2 O
some metabolites
O _
O P
O
O
P
O _
OH
HO
O
P
O _
O _
H 2 O
H 2 O
O O
O P O P O _
O _ O _
“high-energy” phosphoanhydride
bond found in molecules
such as ATP
also written as
O P P
Panel 2.01b