Lecture 5: Lipids and Carbohydrates Craig Kasper Fish Nutrition

Part 1: Lipid Characteristics n Lipid = a compound that is insoluble in water, but soluble in an organic solvent (e.g., ether, benzene, acetone, chloroform) n “lipid” is synonymous with “fat”, but also includes phospholipids, sterols, etc. n chemical structure: glycerol + fatty acids

Lipid Molecule

Nutritional Uses of Lipids n We already know that lipids are concentrated sources of energy (9.45 kcal/g) n other functions: n 1) provide means whereby fat-soluble nutrients (e.g., sterols, vitamins) can be absorbed by the body n 2) structural element of cell, subcellular components n 3) components of hormones and precursors for prostaglandin synthesis

Lipid Classes n simple: FA’s esterified with glycerol n compound: same as simple, but with other compounds also attached n phospholipids: fats containing phosphoric acid and nitrogen (lecithin) n glycolipids: FA’s compounded with CHO, but no N n derived lipids: substances from the above derived by hydrolysis n sterols: large molecular wt. alcohols found in nature and combined w/FA’s (e.g., cholesterol)

Saturated vs. Unsaturated Fatty Acids n saturated: the SFA’s of a lipid have no double bonds between carbons in chain n polyunsaturated: more than one double bond in the chain n most common polyunsaturated fats contain the polyunsaturated fatty acids (PUFAs) oleic, linoleic and linolenic acid n unsaturated fats have lower melting points n stearic (SFA) melts at 70 o C, oleic (PUFA) at 26 o C

Fatty Acids Commonly Found in Lipids

Saturated vs. Unsaturated Fats n saturated fats tightly packed, clog arteries as atherosclerosis n because of double bonds, polyunsaturated fats do not pack well -- like building a wall with bricks (sat.) vs. irregular-shaped objects (unsat.) n plant fats are much higher in PUFA’s than animal fats

Saturated vs. Unsaturated FA’s Plant vs. Animal Fat

Lipid Digestion/Absorption n Fats serve a structural function in cells, as sources of energy, and insulation n the poor water solubility of lipids presents a problem for digestion: substrates are not easily accessible to digestive enzymes n even if hydrolyzed, the products tend to aggregate to larger complexes that make poor contact with the cell surface and aren’t easily absorbed n to overcome these problems, changes in the physical state of lipids are connected to chemical changes during digestion and absorption

Lipid Digestion/Absorption Five different phases: ¶ hydrolysis of triglycerides (TG) to free fatty acids (FFA) and monoacylglycerols · solubilization of FFA and monoacylglycerols by detergents (bile acids) and transportation from the intestinal lumen toward the cell surface ¸ uptake of FFA and monoacylglycerols into the cell and resynthesis to triglyceride Í packaging of TG’s into chylomicrons Î exocytosis of chylomicrons into lymph

Enzymes Involved in Digestion of Lipids n lingual lipase: provides a stable interface with aqueous environment of stomach n pancreatic lipase: major enzyme affecting triglyceride hydrolysis n colipase: protein anchoring lipase to the lipid n lipid esterase: secreted by pancreas, acts on cholestrol esters, activated by bile n phospholipases: cleave phospholipids, activated by trypsin

What about Bile??? n These are biological detergents synthesized by the liver and secreted into the intestine n they form the spherical structures (micelles) assisting in absorption n hydrophobic portion (tails of FA) are located to the inside of the micelle, with heads (hydrophillic portion) to the outside n they move lipids from the intestinal lumen to the cell surface n absorption is by diffusion (complete for FA and monoglycerides, less for others)

Factors Affecting Absorption of Lipids n amount of fat consumed (  fat=  digestion=  absorption) n age of subject (  age =  digestion) n emulsifying agents (  digestion =  absorption) n chain length of FA’s (> 18C =  digestibility) n degree of saturation of FA (  sat =  digestibility) n overheating and autooxidation (rancidification (rot) at double bond) n optimal dietary calcium = optimal FA absorption (high Ca =  absorption)

Lipid Metabolism/Absorption n short chain FA’s are absorbed and enter the portal vein to the liver n those FA’s with more than 10 carbons are resynthesized by the liver to triglycerides n they are then converted into chylomicrons and pass to the lymphatic system n some FA’s entering the liver are oxidized for energy, others stored n blood lipids: 45% P-lipids, 35% triglycerides, 15% cholestrol esters, 5% free FA’s

Lipid Digestion/ Absorption

Characteristics of Fat Storage n Most of the body’s energy stores are triglycerides n storage is in adipose, source is dietary or anabolism (synthesis) from COH or AA carbon skeletons n remember obesity? n adipose can remove FA’s from the blood and enzymes can put them back

Fatty Acid Nomenclature n Nomenclature reflects location of double bonds n also used are common names (e.g., oleic, stearic, palmitic) n linoleic is also known as 18:2 n-6 n this means the FA is 18 carbons in length, has 2 double bonds, the first of which is on the 6th carbon n arachidonic = 20:4 n-6

What’s in a Name?? Fatty Acid Nomenclature

Essential Fatty Acids n Only recently determined as essential (1930) n body can synthesize cholesterol, phospholipids n research: same as AA’s but via addition (EFA’s added improved growth, NEFA’s didn’t) n requirement determined by depleting fat reserves of subject animal: difficult

Essential Fatty Acids (fish) n Most NEAA found in marine food webs n Essential fatty acids (to date): –linoleic (18:2 n-6; terrestrials; fish - not really) –linolenic (18:3 n-3; terrestrials; fish) –arachidonic (20:4 n-6; marine maybe) –eicosopentaenoic acid (20:5 n-3, marine) –docosohexaenoic (22:6 n-3, marine) n Why? Because elongation beyond 18 carbons is very difficult in marine fish (lack pathways) n actual EFA requirement is a matter of whether the fish is FW/SW or WW/CW

Essential Fatty Acids (most animals) n salmonids need n-3 FA’s for membrane flexibility in cold water (why does this work?) n trout can elongate and desaturate n-3 FA’s n Linoleic acid (18:2 n-6) is the most essential n addition of arachidonic is also helpful in deficient diets, but can be synthesized from linoleic (maybe sparing effect) n EFA’s, like EAA’s, must be dietary

Essential Fatty Acids LINOLEICCH 3 (CH 2 ) 4 CH=CHCH 2 CH=CH(CH 2 ) 7 COOH 18:2 n-6 LINOLENICCH 3 CH 2 CH=CHCH 2 CH=CHCH 2 CH=CH(CH 2 ) 7 COOH 18:3 n-3 EICOSOPENTAENOIC ACID CH 3 CH 2 CH=CHCH 2 CH=CHCH 2 CH=CHCH 2 CH=CHCH 2 CH=CH(CH 2 ) 3 COOH 20:5 n-3 DOCOSOHEXAENOIC ACID - YOU CAN DO THIS ONE!

Lipids as Crustacean Energy Sources n Largely, n-6 FA’s (linoleic) used for energy n as temperature drops, requirement for monounsaturated and PUFA’s increases n change in temperature = change in diet n cold water species = increased dietary HUFA’s n maturation animals: increased requirement for 20:4 n-6, 20:5 n-3 and 22:6 n-3 for proper spawning

Part 2: Carbohydrate Characteristics From: Lovell; D’Abramo et al.

General Comments n Carbohydrates often written as “COH” n much of what we need to know about them, besides their structure, was covered in “Bioenergetics, Parts 1&2” n here, we cover structure

Carbohydrate Structure n Basic chemical structure consists of sugar units n found as aldehydes or ketones derived from polyhydric alcohols n contain: C, H, O n often shown as aliphatic or linear structures, but exist in nature as ringed structures

Glucose Structure O C-H H- C-OH HO-C-H H-C-OH CH 2 OH O H HO OH H H H Haworth perspective

Carbohydrate Classification n Usually by the number of sugar units in the molecule: –monosaccharides (glucose) –disaccharides (2 units) n maltose (2 glucose units) n sucrose (glucose + fructose) –polysaccharides (long chain polymers of monosaccharides –most important polysaccharides to animals are starch and cellulose

Starch and Cellulose CH 2 OH O H OH H H H CH 2 OH O H OH H H H O OO CH 2 OH O H OH H H CH 2 OH O OH H H H O O H H O starch cellulose

Starch and Cellulose n Starch contains  -D-glucose linkage n Cellulose has a  -D-glucose linkage n we store starch in muscle tissues as glycogen, peeled off by enzymes when needed n cellulose is primary component of plant tissue, largely indigestible to monogastrics n must have enzyme, “cellulase”