Biochemistry 3 Test

is the sequence of reactions that metabolizes one molecule of glucose to two molecules of pyruvate with the concomitant net production of two molecules of ATP. This process is anaerobic (i.e., it does not require O2) because it evolved before substantial amounts of oxygen accumulated in the atmosphere.

anaerobically to lactatelactic acid fermentation) or ethanol (alcoholic fermentation).

the pancreatic enzyme a-amylase and to a lesser extent by salivary α-amylase.

the α-1,4 bonds of starch and glycogen, but not the α-1,6 bonds

maltose into two glucose molecules, whereas α-glucosidase digests maltotriose and any other oligosaccharides that may have escaped digestion by the amylase

common and important fuel. In mammals, glucose is the only fuel that the brain uses under nonstarvation conditions and the only fuel that red blood cells can use at all.

First, glucose is one of several monosaccharides formed from form-aldehyde under prebiotic conditions, and so it may have been available as a fuel source for primitive biochemical systems. Second, glucose has a low tendency, relative to other monosaccharides, to nonenzymatically glycosylate proteins. In their open-chain forms, monosaccharides contain carbonyl groups that can react with the amino groups of proteins to form Schiff bases, which rearrange to form a more stable amino–ketone linkage.

Stage 1 is the trapping and preparation phase. No ATP is generated in this stage. Stage 1 begins with the conversion of glucose into fructose 1,6-bisphosphate, which consists of three steps: a phosphorylation, an isomerization, and a second phosphorylation reaction. The strategy of these initial steps in glycolysis is to trap the glucose in the cell and form a compound that can be readily cleaved into phosphorylated three-carbon units. Stage 1 is completed with the cleavage of the fructose 1,6-bisphosphate into two three-carbon fragments. These resulting three-carbon units are readily interconvertible. In stage 2,

ATP is harvested when the three-carbon fragments are oxidized to pyruvate.

1) glucose 6-phosphate cannot pass through the membrane because it is not a substrate for the glucose transporters, and (2) the addition of the phosphoryl group acts to destabilize glucose, thus facilitating its further metabolism. The transfer of the phosphoryl group from ATP to the hydroxyl group on carbon 6 of glucose is catalyzed by hexokinase.

are enzymes that catalyze the transfer of a phosphoryl group from ATP to an acceptor.

Lactate formed by active muscle is converted into glucose by the liver. The cycle shifts part of the metabolic burden of active muscle to liver. Thus, the liver restores the level of glucose necessary for active muscle cells, which derive ATP from the glycolytic conversion of glucose into lactate.

　 "Deficiency" is not quite the appropriate term because a decrease in lactase is normal in the course of development in all mammals.

　 Lactose is a good energy source for microorganisms in the colon, and they ferment it to lactic acid while generating methane (CH4) and hydrogen gas (H2). The gas produced creates an uncomfortable feeling of gut distension and the annoying problem of flatulence. The lactate produced by the microorganisms is osmotically active and draws water into the intestine, as does any undigested lactose, resulting in diarrhea.

catalyzes the transfer of a phosphoryl group from ATP to a variety of six-carbon sugars (hexoses), such as glucose and mannose. Hexokinase, like adenylate kinase (Section 9.4) and all other kinases, requires Mg2+ (or another divalent metal ion such as Mn2+) for activity. The divalent metal ion forms a complex with ATP.