Carbohydrate, Lipid, and Protein Metabolism Combined Chapters Carbohydrate, Lipid, and Protein Metabolism

Stage 1: Digestion of Carbohydrates In the mouth, salivary amylase hydrolyzes a-glycosidic bonds in polysaccharides to give smaller polysaccharides (dextrins), maltose, and some glucose. In the small intestine, pancreatic amylase hydrolyzes dextrins to maltose and glucose. The disaccharides maltose, lactose, and sucrose are hydrolyzed to monosaccharides. The monosaccharides enter the bloodstream for transport to the cells.

Summary of carbohydrate digestion in the human body.

Metabolism Section of the small intestine, showing its folds and the villi that cover the inner surface of the folds. © Ed Reschke / Peter Arnold, Inc.

Glycolysis: Splitting Sugar In Stage 2, the metabolic pathway called glycolysis degrades glucose (6C) obtained from digestion to pyruvate (3C)

Glycolysis: Energy-Investment In reactions 1-5 of glycolysis: Energy is used to add phosphate groups to glucose and fructose Glucose is converted to two three-carbon molecules

Glycolysis: Energy-Production In reactions 7 and 10, the hydrolysis of triose phosphates generates four ATP molecules

phosphoglucoisomerase Step 1: Formation of Glucose 6-phosphate Step 2: Formation of Fructose 6-Phosphate ATP ADP Step 1 Hexokinase Step 2 phosphoglucoisomerase

Step 3: Formation of Fructose 1,6-bisphosphate Step 4: Formation of Triose Phosphates ATP ADP Step 3 phosphofructokinase C O H 2 P + 1 4 1 Step 4 6 5 2 2 5 6 4 3 Aldolase 3 dihydroxyacetonephosphate glyceraldehyde 3-phosphate

Step 5: Isomerization of Triose Phosphates 2 Triosephosphate isomerase By the end of Step 5, we have: 1. Used two molecules of ATP so we can 2. Break Glucose (6C) into 2 3C units of glyceraldehyde 3-phosphate

phosphoglycerokinase Step 6: Formation of 1,3-Bisphosphoglycerate Step 7: Formation of 3-Phosphoglycerate Step 6 + 2NAD+ +2Pi 2 2 + 2NADH + 2H+ Glyceraldehyde 3-phosphate dehydrogenase Step 7 2ADP 2ATP 2 2 phosphoglycerokinase

Step 8: Formation of 2-Phosphoglycerate Step 9: Formation of Phosphoenolpyruvate phosphoglyceromutase Step 9 2 2 enolase +2H2O

Step 10: Formation of Pyruvate 2ADP 2ATP 2 2 pyruvate kinase By the end of Step 10, we have: 1. Generated 4 ATP molecules (2 net)

Glycolysis: Overall Reaction Glycolysis generates 2 ATP and 2 NADH Two ATP are used in energy-investment to add phosphate groups to glucose and fructose-6-phosphate Four ATP are formed in energy-generation by direct transfers of phosphate groups to four ADP. Glucose + 2ADP + 2Pi + 2NAD+  2Pyruvate + 2ATP + 2NADH + 4H+

Carbohydrate Metabolism

Regulation of Glycolysis Reaction 1 Hexokinase is inhibited by high levels of glucose-6-phosphate (feedback) Reaction 3 Phosphofructokinase, an allosteric enzyme, is inhibited by high levels of ATP and activated by high levels of ADP and AMP Reaction 10 Pyruvate kinase, another allosteric enzyme is inhibited by high levels of ATP or acetyl CoA

Pathways for Pyruvate When oxygen is present in the cell, pyruvate from glycolysis is decarboxylated to produce acetyl CoA (enters TCA cycle) and CO2 Pyruvate + HS-CoA + NAD+  acetyl CoA + CO2 + NADH + H+

Lactate Formation When oxygen is not available, pyruvate is reduced to lactate, which replenishes NAD+ to continue glycolysis Pyruvate + NADH + H+  lactate + NAD+

Lactate in Muscles Under anaerobic conditions (strenuous exercise): Oxygen in the muscles is depleted Lactate accumulates in the muscles Muscles tire and become painful Rest is needed to repay the oxygen debt and to reform pyruvate in the liver

Fermentation Fermentation: Occurs in anaeobic microorganisms such as yeast Decarboxylates pyruvate to acetaldehyde, which is reduced to ethanol. Regenerates NAD+ to continue glycolysis Pyruvate + NADH + H+  ethanol + NAD+ + CO2

All three of the common fates of pyruvate from glycolysis provide for the regeneration of NAD+ from NADH.

Carbohydrate Metabolism Structural relationships among glycerol and acetone and the C3 intermediates.

Metabolism Entry points for fructose and galactose into the glycolysis pathway. Note: Glycolysis functions in the cytosol.

NADH SHUTTLE The dihyroxyacetone phosphate-glycerol 3-phosphate shuttle. Cytosolic NADH can not cross mitochondrial membrane. Shuttle brings cytosolic electrons into mitochondria in form of mitochondrial FADH2

Carbohydrate Metabolism The processes of glycogenesis, storage of glucose and glycogenolysis, liberating glucose, are contrasted.

Cori Cycle The Cori Cycle

Metabolism

Carbohydrate Metabolism Saturn Stills / SPL / Photo Researchers A diabetic giving himself a blood glucose test.

Lipid Metabolism The events that must occur before triacylglycerols can reach the bloodstream through the digestive process.

Lipid Metabolism Chylomicron, a type of lipoprotein

Lipid Metabolism Structural characteristics of the adipose cell.

Lipid Metabolism Fatty acids are transported across the inner mitochondrial membrane in the form of acyl carnitine.

Reactions of the fatty acid spiral for an 18:0 fatty acid.

Lipids cont’d

Protein Metabolism Summary of protein digestion in human body.

Protein Metabolism

Protein Metabolism Possible fates for amino acid degradation products.

Protein Metabolism Key compounds in the transamination / oxidative deamination process include three keto acid/amino acid pairs.

The four-step urea cycle.

Protein Metabolism Fates of the carbon skeletons of amino acids.

Protein Metabolism The starting materials for the biosynthesis of the 11 nonessential amino acids.

Human body response to feasting, fasting, and starving.