Nutrition and Metabolism

Metabolism

• Recall: metabolism = all chemical reactions occurring in the body.– Reactants = substances that “enter” a chemical reaction– Products = substances that are formed by a chemical

reaction• Metabolic pathways = specific series of reactions in

the body– Begin with specific reactants and make specific produts.– Occur over many steps.– Each step is triggered by a catalyst called an enzyme.

Metabolism cont’d• Enzymes– Protein molecules which function as

catalysts to speed up rate of chemical reactions

– In a reaction using an enzyme, the “reactant” is called a substrate.

Digestive Enzymes - Overview

• Much of digestion relies on the use of specific digestive enzymes.

• Many cause hydrolytic reactions to occur.– Substrate is broken down by a reaction with

water.– Large nutrients get broken into smaller molecules.

• Digestive enzymes need an optimal pH to work properly.– Maintains shape of the enzyme.

Major Digestive enzymes

• Salivary amylase – Catalyzes the reaction: starch + H2O maltose

• Starch is broken down into a smaller sugar – maltose.• Maltose is made up of two glucose • Occurs in the mouth

Major Digestive Enzymes cont’d

• Pepsin – Catalyzes the reaction: protein + H2O

peptides • Only active when pH < 2• Occurs in the stomach in presence of HCl (strong

acid).

Major Digestive enzymes cont’d.

• Pancreatic amylase– Catalyzes the reaction: starch + H2O maltose– Occurs in duodenum (part of the small intestine)• pH must be a little basic (over 7).

– Completes digestion of starches to disaccharides• Disaccharides are molecules made up of 2 sugar

molecules.

Disaccharide examples

Major Digestive enzymes cont’d.

• Trypsin– Catalyzes the reaction: protein + H2O peptides– Occurs in duodenum– Produced by pancreas as trypsinogen – an inactive

form (switched off).– Activated (switched on) in duodenum by another

enzyme called enterokinase.

Major Digestive Enzymes cont’d

• Lipase– Catalyzes the

reaction: fats + H2O glycerol + 3 fatty acids

– Occurs in duodenum

Major Digestive enzymes cont’d.

• Peptidases– Catalyze reaction: peptides + H2O amino acids

• Occurs in small intestine

• Maltase– Catalyzes reaction: maltose + H2O 2 Glucose

• Occurs in small intestine

• Each disaccharide has its own enzyme (like maltase)– Examples include lactase and sucrase– Lactase = enzyme that breaks down lactose; sucrase =

enzyme that breaks down sucrose.

Major Digestive enzymes cont’d.

• Table 14.3

Digestive Enzymes – Conditions Required

• Environmental conditions must be optimum– Changes to conditions harm the enzymes.

• Conditions include:• Warm temperature

– Most function well at normal body temperature– Extreme heat causes irreversible shape change – denaturing.

• Correct pH– Each enzyme has its own optimal pH– Most function at near neutral pH (7).– Others prefer acidic (lower pH) or basic (higher pH) conditions.