CH 461 Study Guide - Summer 2018, Comprehensive Midterm Notes - Protein, Enzyme, Adenosine Triphosphate
CH 461
MIDTERM EXAM
STUDY GUIDE
Fall 2018
Dietary Fat Absorption
Energy production from triacylglycerols starts with their hydrolysis into free fatty acids
and glycerol. Enzymes called lipases, which catalyze the reaction, carry out this
hydrolysis.
The reaction releases the three fatty acids and glycerol. An intestinal carrier absorbs the
glycerol, which will eventually rejoin with fatty acids in the intestinal cells.
The body must absorb the fatty acids released by the lipases by a rather more involved
mechanism. Fatty acids are poorly soluble in water, although they are more soluble than
triacylglycerols. Lipids of whatever kind tend to form droplets. Protein enzymes are
water‐ soluble and therefore cannot gain easy access to the lipid droplet. To be
digested, lipids must be emulsified into small droplets, which have a larger surface area.
In other words, the hydrophobic interactions forcing the lipids into larger droplets must
be overcome. The molecules that carry out this function are called bile salts or bile
acids. Metabolically, the liver creates them and secretes them into the gall bladder,
from where they are pumped into the duodenum.
Bile salts are derived from cholesterol and are a major end product of cholesterol
metabolism. They are powerful detergents, with a large, hydrophobic component and a
carboxylic acid end‐group that is negatively charged at the pH characteristic of the small
intestine. The hydrophobic component of the bile acid will associate above a specific
concentration (termed the critical micelle concentration, or CMC) to form disc‐
shaped micelles, that is, droplets. Common bile salts have CMCs in the 2 to 5
(millimolar) range. The micelles in the gut contain dietary lipids (triacylglycerol,
cholesterol, and fatty acids) as well as bile salts, and are termed mixed micelles for
that reason.
The bile salts form the edge of the micelle and also appear, in fewer numbers,
dispersed throughout the inside of the micelle. The lipids exist in a bilayer on the inside
of the disc. Bile acids are important for fatty acid absorption. Fat‐soluble vitamins (A, D,
E, and K) absolutely require bile acids for absorption.
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The mixed micelle provides a large surface area for the action of pancreatic lipase,
which is responsible for the majority of digestive action. Pancreatic lipase uses a
cofactor, a small protein called colipase, which binds both to lipase and to the micelle
surface. The action of lipase leads to free fatty acids that are slightly soluble in the
aqueous phase of the gut. For the most part, the cells of the small intestine absorb
these free fatty acids; the bacteria in the large intestine metabolize and/or absorb those
that pass through the small intestine. The bile salts are reabsorbed in the last third of
the small intestine.
Bile‐acid metabolism explains the ability of certain kinds of dietary fiber to help lower
serum cholesterol. A molecule of bile acid circulates through the liver and intestine five
or more times before finally being eliminated. Soluble fiber (such as that found in oat
bran) binds bile acids, but itself cannot be absorbed. Therefore, fiber‐bound bile acids
are eliminated in the stool. Because bile acids derive from cholesterol, synthesizing
more bile acid drains the body's stores of cholesterol, which leads to a reduction in
serum cholesterol, and therefore, to a lower risk of coronary artery disease. Eating oat
fiber cannot overcome an excessive dietary cholesterol consumption, of course. In other
words, consuming excessive amounts of well‐marbled steak and expecting to overcome
the effects by eating a bran muffin would be foolish.
Lipids in the bloodstream
Free fatty acids are transported as complexes with serum albumin. Cholesterol, triacylglycerols,
ad phospholipids are trasported as protei‐lipid coplexes called lipoproteins.
Lipoproteins are spherical, with varying amounts and kinds of proteins at their surfaces. The
protein components, of which at least ten exist, are called apolipoproteins. Lipoproteins are
classified in terms of their density.
The lightest and largest of the apolipoproteins are the chylomicrons, which are less
dense than water by virtue of their being composed of more than 95 percent lipid by
weight (remember that oils float on water because they are less dense than water).
Triacylglycerols make up most of the lipid component of chylomicrons, with small
amounts of phospholipid and cholesterol. Chylomicrons contain several kinds of
apolipoproteins.
Very‐Low‐Density Lipoproteins (VLDL) are less dense than chylomicrons. They contain
more protein, although lipids (fatty acids, cholesterol and phospholipid, in that order) still
make up 90 to 95 percent of their weight. Low‐density lipoproteins (LDLs) are about 85
percent lipid by weight and contain more cholesterol than any other kind of lipid. VLDL
and LDL contain large amounts of Apolipoprotein B. The VLDL and LDL are sometimes
referred to as bad cholesterol because elevated serum concentrations of these
lipoproteins correspond with a high incidence of artery disease (stroke and heart
disease). The LDLs carry cholesterol and fatty acids to sites of cellular membrane
synthesis.
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