Glucose-6-phosphate dehydrogenase (G6PD)
crose). It is also the repeating monomer from which some polysaccharides (e.g., starch, cellulose, and gly- cogen) are made.
glucose-6-phosphate dehydrogenase (G6PD) an enzyme catalyzing the conversion of glucose-6-phos- phate to 6-phosphogluconate. G6PD from human erythrocytes is a dimer consisting of identical sub- units, each containing 515 amino acids. About 400 genetic variants of this enzyme are known. The four most common forms of the enzyme are
glucose-6-phosphate dehydrogenase deficiency (G6PD) the most common disease-producing en- zyme deficiency of humans. It affects about 400 mil- lion humans. The gene (Gd+) encoding G6PD re- sides at Xq28. The red blood cells of males with the A− or M forms of the enzyme have a reduced life span, and exposure to antimalarial drugs such as pri- maquine results in life-threatening hemolysis.
As a result of random X chromosome inactivation, fe- males heterozygous for the mutant G6PD gene have some erythrocytes with the normal enzyme and some with the defective enzyme. Mutant G6PD al- leles are kept in some human populations because of the heterozygote advantage (q.v.) occurring during infections by Plasmodium falciparum. Gd+/Gd fe- males show lower levels of parasitemia than Gd+/ Gd+ females. See Appendix C, 1962, Beutler et al., dosage compensation, favism, malaria. glucose-sensitive operons bacterial operons whose activity is inhibited by the presence of glucose. This lowers the level of cyclic AMP, thereby blocking a required positive control signal.
glucosylceramide lipidosis Gaucher disease (q.v.). glume a chaffy bract, pairs of which enclose the base of grass spikelets. glutamic acid See amino acid. glutamine See amino acid. glutathione a tripeptide containing glutamic acid, cysteine, and glycine and capable of being alter- nately oxidized and reduced. Glutathione plays an important role in cellular oxidations. gly glycine. See amino acid. glycerol a trihydric alcohol, that combines with fatty acids to form fats (q.v.).
Glycine max the soybean, a legume native to China. Its haploid chromosome number is 20, and its genome size is 1.1 gigabases. The bean is the source of oil used in industry and protein used in human and livestock consumption. The soft, cheese- like bean curd is known as tofu or tubu. The soybean is an example of a nodulating legume (q.v.). See Ap- pendix A, Plantae, Tracheophyta, Angiospermae, Di- cotyledonae, Leguminales; Appendix E, Rhizobium. glycogen a soluble polysaccharide (see formula on page 165) built up of numerous glucose molecules. Carbohydrate is stored as glycogen by vertebrates, especially in liver and muscles. glycogenesis glycogen synthesis from carbohy- drates. See glyconeogenesis. glycogenolysis the liberation of glucose from gly- cogen. glycogenosis glycogen storage disease (q.v.). glycogen storage disease any of a group of con- genital and familial disorders characterized by the deposition of either abnormally large or abnormally small quantities of glycogen in the tissues. Anderson, Forbes, von Gierke, Hers, McArdles, and Pompe dis- eases are examples.
glycolipid a lipid containing carbohydrate. glycolysis the sequential series of anaerobic reac- tions diagrammed on page 186 found in a wide vari- ety of tissues that constitutes the principal route of carbohydrate breakdown and oxidation. The process starts with glycogen, glucose, or fructose and ends with pyruvic or lactic acids. The conversion of a molecule of glucose to two molecules of pyruvic acid generates two ATP molecules. Under aerobic conditions, the yield is eight ATP molecules. The pyruvic acid formed is broken down by way of the citric acid cycle (q.v.). See pentosephosphate path- way. glycolytic participating in glycolysis. glyconeogenesis See gluconeogenesis. glycophorin A a protein containing acids that is encoded by the GYPA gene. It spans the plasma membrane of the human red blood cell once and presents its amino terminal end to the extracel- lular surface. There are two polymorphic versions of glycophorin A in humans.
The only differences are at positions 1 and 5 in the extracellular portion of the molecule. The M antigen contains serine and glycine at positions 1 and 5 in the chain, while the N antigen has leucine and glutamic acid at these po- sitions. See MN blood groups. glycoprotein an organic molecule consisting of a protein covalently linked to one or more carbohy- drate molecules. The carbohydrate components of glycoproteins are usually small sugars, which attach to the protein post-translationally at either an aspar- agine (N-glycans), or at a serine or threonine (O-gly- cans). The attachment of the carbohydrate(s) is thought to affect protein folding, its stability and physical properties, and its recognition by other molecules. See amino acids, carbohydrate, glycosyla- tion, post-translational processing, mucoprotein. glycoside a compound yielding a sugar upon enzy- matic hydrolysis. glycosidic bonds the bonds coupling the mono- saccharide subunits of a polysaccharide.
glycosome a microbody (q.v.), found in parasitic protozoa like trypanosomes, which contains most of the enzymes functioning in the conversion of glu- cose to 3-phosphoglyceric acid. See glycolysis. glycosylation addition of one or more sugars to other molecules such as lipids and proteins; these molecules so modified are called glycolipids and glyco- proteins, respectively. Glycoproteins appear to be universal in eukaryotic cells but are rare in or absent from bacteria. Glycosylation of proteins occurs within the lumen of the endoplasmic reticulum (ER) by glycosyltransferase enzymes. These glycoproteins are transported by clathrin-coated vesicles to the Golgi apparatus, where some sugars are removed and new ones added. This last phase of modification is termed terminal glycosylation to distinguish it from core glyco- sylation that occurs in the ER. In most glycoproteins, sugars are usually added to the hydroxy-group oxy- gen (O-linked) of serine or threonine; N-linked sug- ars are attached to amide nitrogens of asparagine. Most proteins in the cytosol or nucleus are not gly- cosylated. See oligosaccharide. glyoxylate cycle an alternative to the citric acid cycle. This series of metabolic reactions is catalyzed by enzymes localized to glyoxysomes (q.v.).
The cy- cle plays an important role in the photorespiration of plants and in the utilization of fat reserves by seedlings. See peroxisomes. glyoxysome a membrane-bound organelle, found in germinating seeds and other plant tissues; it con- tains enzymes of the glyoxylate cycle (q.v.). See mi- crobody. Glyptotendipes barbipes a midge possessing ex- ceptionally large polytene chromosomes in the sali- vary glands of larvae and therefore a favorite for cy- tological studies.