Instant Notes: Plant Biology

Transaminationreactions involve the transfer of an amino group from one

compound to another. For instance, asparagine synthase(AS;Fig. 2) converts

aspartate and glutamine to asparagine and glutamate. Asparagine contains a

high quantity of nitrogen (2N) for each carbon (4C) present, compared with

glutamate (1N:5C) or glutamine (2N:5C). Expression of the AS gene is repressed

(inhibited) by high light and high carbohydrate production, so the plant makes

nitrogen compounds with low quantities of nitrogen. When light is low and

carbohydrate is scarce, carbon is conserved by the synthesis of compounds with

a much higher nitrogen content.

Plants synthesize a wide variety of lipids, including: membrane lipids, in which

long-chain fatty acids, commonly 16–18 carbons in length, are esterified to

glycerol; the waxes of the cuticle and suberin of the endodermis; and the

storage lipids of seeds (most commonly triacylglycerols).

Plants use acetyl CoA as the basic building block for assembling long-chain

fatty acids. Acetyl CoA is produced by pyruvate dehydrogenase present both

in plastids and mitochondria. In the chloroplast, pyruvateis produced from

3-phosphoglycerate from the Calvin cycle; in non-photosynthetic tissue, it origi-

nates from glycolysis. Synthesis of glycerolipids occurs in two stages: addition

of the fatty acid chains to glycerol-3-phosphate and the addition of a head

group. Glycerol-3-phosphate is derived in stages originating from glyceralde-

hyde-3-phosphate. In photosynthetic tissue, the chloroplast is the chief source of

fatty acids, which may be used directly in the chloroplast or exported to the

cytoplasm. Plant cells can synthesize glycerolipids at the endoplasmic

reticulum (ER) and in mitochondria as well as in plastids; all three of these

organelles are believed to be inter-connected and lipids traffic between them

and to the other cellular membranes.

Seeds make triacylglycerols, which are storage oils, and frequently very

important human foods. Up to 60% of the dry weight of a seed may be in the

form of storage oils. Triacylglycerols consist of a glycerol to which three fatty

acid chains have been added (Fig. 3). They are stored in oil bodies, small lipid

droplets with a surrounding coating of lipid and protein. The oil is first synthe-

sized in the ER.

Lipid
biosynthesis

156 Section J – Metabolism

Glutamate Oxaloacetate Aspartate 2-oxoglutarate

Glutamine ++Aspartate Asparagine Glutamate

COOH

HC NH 2

CH 2

CH 2

CO–

O

NH 2

COOH

CH 2

CO–

O

C O

COOH

CH 2

CO–

O

CH 2

C O

COOH

CH 2

CO–

O

C

++

AAT

Aspartate

aminotransferase

AS

Asparagine

Synthetase

Fig. 2. Other amino acids are synthesized by transamination reactions which occur in various

organelles.