Biochemistry Biochemistry MS410 MS410

Lecture 1 Lecture 1 Nutrition

Dr. Adil Khalil Dr. Adil Khalil

1

Nutrition, digestion & absorption

• Diet: • metabolic fuels:• carbohydrates and lipids• Protein: for growth and turnover of

tissue proteins • Fiber: for roughage• minerals • vitamins and essential fatty acids

• The polysaccharides, triacylglycerols, and proteins must be hydrolyzed to

their constituent monosaccharides, fatty acids, and amino acids, respectively, before absorption and utilization.

• Under nutrition leads to: • impaired growth, • defective immune systems • excessive food consumption

(especially of fat), leading to: • Obesity• development of cardiovascular disease

• Deficiencies of vitamin A• iron• iodine • result in major health concerns, and

deficiencies of other vitamins and minerals are a major cause of ill health.

DIGESTION & ABSORPTIONOF CARBOHYDRATES

• Mouth• Intestine• The digestion of complex

carbohydrates is by hydrolysis to liberate oligosaccharides, then free mono- and disaccharides.

• The hydrolysis of starch by salivary and pancreatic amylases catalyze random hydrolysis of α(1→4) glycoside bonds, yielding dextrins, then a mixture of glucose, maltose.

• The disaccharidases—maltase, sucrase lactase are located on the brush border of the intestinal mucosal cells (monosach).

• lactose intolerance:• Lactose remains in the intestinal

lumen, where it is a substrate for bacterial fermentation to lactate, resulting in discomfort and diarrhea.

• Glucose and galactose are absorbed by a sodium-dependent process.

• Other monosaccharides are absorbed by carrier-mediated diffusion.

DIGESTION & ABSORPTION OF LIPIDS

• The major lipids in the diet are • 1. triacylglycerols and• 2. phospholipids. • These are hydrophobic molecules and

must be hydrolyzed and emulsified to very small droplets (micelles) before

they can be absorbed.

• The fat-soluble vitamins—A, D, E, and K—and a variety of other lipids (including cholesterol) are absorbed dissolved in the lipid micelles. Absorption of the fat-soluble vitamins is impaired on a very low fat

diet.

• Hydrolysis of triacylglycerols is initiated by lingual and gastric lipases that attack the sn-3 ester bond, forming 1,2-diacylglycerols and free fatty acids, aiding emulsification.

• Pancreatic lipase is secreted into the small intestine It is specific for the primary ester links—ie, positions 1 and 3 in triacylglycerols—resulting in 2-monoacylglycerols and free fatty acids as the major end-products of luminal triacylglycerol digestion

• Monoacylglycerols are hydrolyzed with difficulty to glycerol and free fatty acids, so that less than 25% of ingested triacylglycerol is completely hydrolyzed to glycerol and fatty acids

• Bile salts, formed in the liver and secreted in the bile ( emulsification )

• micelles

• DIGESTION & ABSORPTION OF PROTEINS

• Several Groups of Enzymes Catalyze the Digestion of Proteins

• There are two main classes of proteolytic digestive enzymes:

• proteases: 1. Endopeptidases hydrolyze peptide bonds between specific amino acids throughout the molecule. They are the first enzymes to act, yielding a larger number of smaller

fragments, eg, pepsin in the gastric juice and trypsin, chymotrypsin, and elastase secreted into the small intestine by the pancreas.

• 2. Exopeptidases, Carboxypeptidases,

• aminopeptidases

• Dipeptides, which are not substrates for exopeptidases, are hydrolyzed in the brush border of intestinal

mucosal cells by dipeptidases.

• The proteases are secreted as inactive zymogens:

• Pepsinogen is activated to pepsin trypsinogen, the precursor of trypsin

• chymotrypsinogen to chymotrypsin, proelastase to elastase, procarboxypeptidase to carboxypeptidase,

• and proaminopeptidase to aminopeptidase.

• The end product is a mixture of free amino acids, di- and tripeptides, and oligopeptides, all of which are absorbed.

• Free amino acids are absorbed across the intestinal mucosa by sodium-dependent active transport.

• Dipeptides and tripeptides enter the brush border of the intestinal mucosal cells, where they are hydrolyzed to free amino acids.

• Relatively large peptides may be absorbed intact, either by uptake into mucosal epithelial cells or by passing between epithelial cells.

• Many such peptides are large enough to stimulate antibody formation— this is the basis of allergic reactions to foods

DIGESTION & ABSORPTIONOF VITAMINS & MINERALS

• fat-soluble vitamins are absorbed in lipid micelles

• water-soluble vitamins and most minerals are absorbed from the small intestine

• 1. by active transport• 2. by carrier-mediated diffusion • Vitamin B12: intrinsic factor; • calcium absorption: vitamin D; • zinc absorption requires a zinc-binding

ligand secreted by the exocrine pancreas;• absorption of iron is limited.

Calcium Absorption• vitamin D: Synthesis of the intracellular

calcium binding protein, calmodulin.• High concentrations of fatty acids in

the intestinal lumen can also reduce calcium absorption by forming insoluble calcium salts

• high intake of oxalate can sometimes cause deficiency, since calcium oxalate

is insoluble

Iron Absorption

• Inorganic iron is absorbed only in the Fe2+ (reduced) state, and for that reason the presence of reducing agents

will enhance absorption (vitamin C)• absorption of iron is impaired by

calcium a glass of milk with a meal significantly reduces availability

• Absorption of iron is strictly regulated• iron deficiency is a common problem• overload (hemochromatosis)

• Absorption of iron: strictly regulated. • iron: in intestinal mucosal cells bound

to an intracellular protein, ferritin. • Once the ferritin in the cell is saturated

with iron, no more can enter. • Iron can only leave the mucosal cell if

there is transferrin in plasma to bind to.

• Once transferrin is saturated with iron, any that has accumulated in the mucosal cells will be lost when the cells are shed.

As a result of this mucosal barrier, only about 10% of dietary iron is normally absorbed

ENERGY BALANCE• Food intake in excess of energy

expenditure leads to obesity,• intake less than expenditure leads to

emaciation and wasting, as in marasmus and kwashiorkor

• Marasmus can occur in both adults and children and occurs in vulnerable groups of all populations.

• Kwashiorkor only affects children.• Decreased: immunity, muscle mass,

absorption (intestinal cell regeneration)

• Basal metabolic rate (BMR):• is the energy expenditure by the body

when at rest Under controlled conditions of thermal neutrality, measured at about 12 hours after the last meal, and depends on weight, age, and gender

• Total energy expenditure depends on the basal metabolic rate

• The decrease in BMR with increasing age, even when body weight remains constant, is due to muscle tissue replacement by adipose tissue, which is metabolically much less active.

• Similarly, women have a significantly lower BMR than men of the same body weight because women’s bodies have more adipose tissue than men.

• children with kwashiorkor:• oedema, associated with a decreased

concentration of plasma proteins.• Enlargement of the liver due to

accumulation of fat

Nitrogen balance

• The state of protein nutrition can be determined by measuring the dietary intake and output of nitrogenous compounds from the body.

• The output of nitrogen from the body is mainly in urea and other compounds in urine.

• undigested protein in feces, • and significant amounts may also be

lost in sweat and shed skin.

• The difference between intake and output of nitrogenous compounds is known as nitrogen balance.

• Equilibrium• Negative• positive

• equilibrium when intake equals output, and there is no change in the total body content of protein.

• Positive nitrogen balance:• Growing children, pregnancy, recovery

from illness.• Negative: late cancers, malnutrition,

surgery, burn.

Minerals• inorganic substances• formation of structural components and

energy transfer• maintain osmotic pressure and fluid balance• activity of nervous and muscular systems• Macrominerals:calcium, phosphorus,

potassium, sodium, chloride, magnesium, sulfur

• Microminerals: cobalt, copper, iodine, iron, manganese, selenium, zinc

• sodium, potassium, chloride• calcium, phosphorus magnesium

and zinc• excess excreted in urine• over-supplementation

– one mineral prevents absorption of another

– zinc inhibits calcium and copper absorption

– calcium inhibits zinc absorption– manganese interferes with iron

absorption

• calcium– 35% of bone structure– essential for muscle contraction– Blood clotting– Action potential– Activation of enzymes– Release of neurotransmitters

• phosphorus– 15% of skeleton– require for energy transfer reaction -

ATP and ADP– calcium:phosphorus ratio 1:1

• potassium– major intracellular cation– maintain osmotic pressure and acid-

base balance

• sodium– major extracellular cation– maintain acid base balance and osmotic

regulation• chloride

– extracellular anion– acid base balance and osmotic

regulation

• magnesium– .05% of body mass

• 60% for skeleton– activator of enzyme

• iron– constituent of hemoglobin– anemia - iron deficiency

• selenium– component of enzyme glutathione

peroxidase• cell membrane damage

– Oxidation of polyunsaturated fatty acids