12 UNITY AND DIVERSITY IN BIOCHEMISTRY 



If the two functional groups are 1 , 4 or 1 , 5 to each other they will form 

 an internal ester or a lactone. 



CH,— CH2— CH2— CHa CH,— CH,— CHo— CHo 



OH COOH O CO 



1 . Monohydroxy-monoacids 



These substances frequently appear during the process of the breaking 

 down of cellular nutrients. 



The simplest is glycoUic acid CH,OH.COOH. 



Lactic acid exists as two position isomers, primary or ^-lactic acid 

 CH.OH.CHo.COOH, and secondary or a-lactic acid CH3.CHOH.COOH. 

 This last form possesses an asymmetric carbon atom and therefore exists as 

 a laevo- and a dextro- rotator}' form. 



Nature does not contain hydroxy derivatives of butyric acid other than 

 j8-hydroxybutyric acid CH3.CHOH.CH2.COOH. Another interesting 

 monohydroxy-monoacid is hydroxylignoceric acid C22H4;.CHOH.COOH, 

 an acid found combined in certain cerebrosides. 



2. Polyhydroxy-monoacids 



The simplest is glyceric acid CH.,OH.CHOH.COOH. IVIany others may 

 be readily prepared by oxidation of sugars, particularly the aldoses, whose 

 aldehyde group is more readily oxidizable than the keto group of the ketoses 

 (see p. 24). 



3. Monohydroxy-polyacids 



The simplest is tartronic acid COOH. CHOH. COOH, which is a product 

 of the oxidation of glycerol CHoOH.CHOH.CHoOH, of malonic acid 

 COOH.CHo.COOH and of tartaric acid. 



Two other representatives of this group play an important role in the 

 biosphere. They are, malic acid COOH. CHOH. CHo.COOH which yields 

 malonic acid on oxidation and when water is removed is transformed into 

 maleic and fumaric acids, and citric acid COOH. CH2C(0H). (COOH). 

 CH2COOH, which is a monohydrox\'-tribasic acid. Citric acid is trans- 

 formed bv dehvdration at 175° into aconitic acid COOH.CH : C(COOH). 

 CHo.COOH. 



4. Polyhydroxy-polyacids 



A series of these compounds is present in the biosphere ; they are derived 

 from the sugars by oxidation (see p. 24). 



