Metabolism

• In the catabolism of carbohydrates, glycolysis converts glucose into pyruvate, which is then metabolized into acetyl CoA.
• In the catabolism of lipids, fatty acids are converted into thioesters and then cleaved into many acetyl CoA units.
• Amino acids are usually reassembled into new proteins.
• Since excess amino acids are not stored in the body, they can also be catabolized for energy.
• The amino groups (NH2) are converted to urea [(NH2)2C=O], which is excreted in urine.
• The name of an enzyme is often a clue as to the type of reaction it catalyzes:

  
  

  Carboxylase catalyzes the addition of a -COO−(carboxylate).
• The name of an enzyme is often a clue as to the type of reaction it catalyzes:

  
  

  Decarboxylase catalyzes the removal of CO2
• The name of an enzyme is often a clue as to the type of reaction it catalyzes:

  
  

  Dehydrogenase catalyzes the removal of 2 H atoms.
• The name of an enzyme is often a clue as to the type of reaction it catalyzes:

  
  

  Isomerase catalyzes the isomerization of one isomer into another.
• The name of an enzyme is often a clue as to the type of reaction it catalyzes:

  
  

  Kinase catalyzes the transfer of a phosphate.
• The transfer of a phosphate unit from ATP to the fructose 6-phosphate molecule is catalyzed by a kinase enzyme.
• Glycolysis is a linear, 10-step anaerobic pathway that converts glucose into two molecules of pyruvate.
• GLYCOLYSIS

  Steps [1] – [5] comprise the energy investment phase, where 2 ATP molecules are hydrolyzed.
• GLYCOLYSIS

  The 6-carbon glucose molecule is converted into two 3-carbon segments.
• GLYCOLYSIS

  Steps [6] – [10] comprise the energy-generating phase, producing 1 NADH and 2 ATPs for each pyruvate formed.
• GLYCOLYSIS

  Step [1] begins with the phosphorylation of glucose into glucose 6-phosphate, using an ATP and a kinase enzyme.
• GLYCOLYSIS

  Step [2] isomerizes glucose 6-phosphate to fructose 6-phosphate with an isomerase enzyme.
• GLYCOLYSIS

  Step [3] is the phosphorylation of fructose 6-phosphate into fructose 1,6-bisphosphate with a kinase enzyme.
• GLYCOLYSIS

  Overall, the first three steps of glycolysis add 2 phosphate groups and isomerize a 6-membered glucose ring into a 5-membered fructose ring.
• GLYCOLYSIS

  The energy stored in 2 ATP molecules is utilized to modify the structure of glucose for the later steps that generate energy.
• GLYCOLYSIS

  Step [4] cleaves the fructose ring into a dihydroxy-acetone phosphate and a glyceraldehyde 3-phosphate.
• GLYCOLYSIS

  Step [5] isomerizes the dihydroxyacetone phosphate into another glyceraldehyde 3-phosphate.
• GLYCOLYSIS

  Thus, the first phase of glycolysis converts glucose into 2 glyceraldehyde 3-phosphate units.
• GLYCOLYSIS

  In step [6] the aldehyde end of the molecule is oxidized and phosphorylated by a dehydrogenase enzyme and NAD+; this produces 1,3-bisphospho-glycerate and NADH.
• GLYCOLYSIS

  In step [7], the phosphate group is transferred onto an ADP with a kinase enzyme, forming 3-phosphoglycerate and ATP.
• GLYCOLYSIS

  In step [8], the phosphate group is isomerized to a new position in 2-phosphoglycerate.
• GLYCOLYSIS

  In step [9], water is lost to form phosphopenol-pyruvate.
• GLYCOLYSIS

  In step [10], the phosphate is transferred to an ADP, yielding pyruvate and ATP with a kinase enzyme.
• GLYCOLYSIS

  The 2 glyceraldehyde 3-phosphate units are converted into 2 pyruvate units in phase two of glycolysis.
• GLYCOLYSIS

  Overall, the energy-generating phase forms 2 NADHs and 4 ATPs.
• GLYCOLYSIS

  2 ATPs are used in phase one of glycolysis, and 4 ATPs are made in phase two of glycolysis. The net result is the synthesis of 2 ATPs from glycolysis.
• GLYCOLYSISThe 2 NADHs formed are made in the cytoplasm and must be transported to the mitochondria to join the electron transport chain and make ATP.
• GLYCOLYSIS

  The fate of the 2 pyruvate molecules depends on O2 availability.
• Fructose is obtained by the hydrolysis of the disaccharide sucrose, found in sugar beets and sugarcane.
• GLYCOLYSIS with Fructose

  It can be converted by muscle or kidney cells into fructose 6-phosphate and enter glycolysis at step [3].
• GLYCOLYSIS with Fructose

  Or, it can be converted by the liver to glyceraldehyde 3-phosphate and enter glycolysis at step [6].
• GLYCOLYSIS with Galactose

  Galactose is obtained by the hydrolysis of the disaccharide lactose in milk.
• GLYCOLYSIS with Galactose

  Galactose is converted into glucose 6-phosphate and then enters glycolysis in step [2].
• GLYCOLYSIS with Galactose

  Patients with galactosemia lack the enzyme to perform this conversion.
• GLYCOLYSIS with Mannose

  Mannose is obtained from polysaccharides in fruits such as cranberries and currants.