Metabolism• Metabolism involves two main processes, catabolism and

anabolism • Catabolic reactions break down large, complex molecules to

provide smaller molecules and energy (ATP)• Anabolic reactions use ATP energy to build larger molecules

from smaller building blocks

Stages of Catabolism

Catabolic reactions are organized into three stages:

• In Stage 1, digestion breaks down large molecules into

smaller ones that enter the bloodstream

• In Stage 2, molecules enter the cells and are broken down

into two- and three-carbon compounds

• In Stage 3, compounds are oxidized in the citric acid cycle

to provide energy (ATP) for anabolic processes

Stages of Catabolism (Diagram)

Eukaryotic Cell Structure

• Metabolic reactions occur at specific sites within the cell

Components of Eukaryotic Cells

ATP and Energy

• In cells, energy is stored in adenosine triphosphate (ATP)- there are other cellular energy sources, but ATP is the main one

Hydrolysis of ATP• The hydrolysis of ATP to ADP releases 7.3 kcal (31 kJ/mole)

ATP ADP + Pi + 7.3 kcal (31 kJ/mole)• The hydrolysis of ADP to AMP releases 7.3 kcal (31 kJ/mole)

ADP AMP + Pi + 7.3 kcal (31 kJ/mole)

ATP and Muscle Contraction

• Muscle fibers contains filaments of actin and myosin

• When a nerve impulse increases [Ca2+], the filaments slide closer together to contract the muscle

• The hydrolysis of ATP in muscle provides the energy for contraction

• As Ca2+ and ATP decrease, the filaments return to the relaxed position

Coenzyme NAD+

• When a compound is oxidized by an enzyme, 2H as 2H+ and 2e- are removed by a coenzyme, which is reduced

• NAD+ (nicotinamide adenine dinucleotide) participates in reactions that produce a carbon-oxygen double bond (C=O)

• For example, NAD+ participates in the oxidation of ethanol:

Overall Reaction:

OHH

O

+ NADH + H++ NAD+Alcohol

dehydrogenase

Oxidation:

OH

H

O

+ 2H+ + 2e-

Reduction:

NAD+ + 2H+ + 2e- NADH + H+

Structure of Coenzyme NAD+

• NAD+ (nicotinamide adenine dinucleotide) contains ADP, ribose, and nicotinamide (from niacin, B3)

• NAD+ reduces to NADH when the nicotinamide group accepts H+ and 2e-

Coenzyme FAD• FAD participates in reactions that produce a carbon-carbon

double bond (C=C)

Oxidation

—CH2—CH2— —CH=CH— + 2H+ + 2e-

Reduction

FAD + 2H+ + 2e- FADH2

Structure of Coenzyme FAD• FAD (flavin adenine dinucleotide) contains ADP and riboflavin

(vitamin B2)

• FAD reduces to FADH2 when flavin accepts 2H+ and 2e-

Coenzyme A (CoA)

• CoA activates acyl groups, such as the two-carbon acetyl group for transfer to other compounds

• It consists of pantothenic acid (vitamin B5), phosphorylated ADP

and an aminoethanethiol

Digestion of Carbohydrates (Stage 1)

• In the mouth, salivary amylase hydrolyzes -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 in the small intestine

• The monosaccharides enter the bloodstream

- fructose and galactose are transported to the liver, where they are isomerized to glucose

- glucose is transported to cells for metabolism

Overview of Stage 1 of Carbohydrate Catabolism

Glycolysis (Stage 2)

• In Stage 2 of carbohydrate catabolism, the metabolic pathway called glycolysis degrades glucose (6C) obtained from digestion to pyruvate (3C)

• Glycolysis is an anaerobic process that takes place in the cytoplasm

Energy-Investing Phase of GlycolysisIn reactions 1-5 of glycolysis:• Energy is used to add phosphate groups to glucose and fructose• Glucose is converted to two three-carbon molecules

Energy-Producing Phase of Glycolysis

• In reactions 6-10, the hydrolysis of phosphates generates four ATP molecules

• Two NAD+ coenzymes are also reduced

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

Overall Reaction:

Glucose + 2ADP + 2Pi + 2NAD+

2Pyruvate + 2ATP + 2NADH + 4H+

Regulation of Glycolysis• The amount of glucose that goes through glycolysis is

regulated based on relative levels of ATP, ADP and AMP, as well as other glycolysis intermediates

• This regulation takes place at three steps:

- Reaction 1: Hexokinase is inhibited by high levels of glucose-6-phosphate, which prevents the phosphorylation of glucose

- 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