Nutrition Science and Everyday Application - beta v 0.1

322 ALICE CALLAHAN, PHD, HEATHER LEONARD, MED, RDN, AND TAMBERLY POWELL, MS, RDN
Protein Types and Functions
Type Examples Functions
Structure
Enzymes
Hormones
Fluid and acid-
base balance
Transport
Defense
Actin, myosin,
collagen, elastin,
keratin
Amylase, lipase,
pepsin, lactase
Insulin, glucagon,
thyroxine
Albumin,
hemoglobin
Hemoglobin,
albumin,
protein channels,
carrier proteins
Collagen, lysozyme,
antibodies
Give tissues (bone, tendons, ligaments, cartilage, skin, muscles) strength and
structure
Digest macronutrients into smaller monomers that can be absorbed;
performs steps in metabolic pathways to allow for nutrient utilization
Chemical messengers that travel in blood and coordinate processes around
the body
Maintains appropriate balance of fluids and pH in different body
compartments
Carry substances around the body in the blood or lymph; help molecules
cross cell membranes
Protect the body from foreign pathogens
Table 6.2. Protein types and functions
STRUCTURE
More than one hundred different structural proteins have been discovered in the human
body, but the most abundant by far is collagen, which makes up about 6 percent of total
body weight. Collagen makes up 30 percent of bone tissue and comprises large amounts of
tendons, ligaments, cartilage, skin, and muscle. Collagen is a strong, fibrous protein made
up of mostly glycine and proline amino acids. Within its quaternary structure, three protein
strands twist around each other like a rope and then these collagen ropes overlap with
others.
Figure 6.10. Triple-helix structure of collagen
This highly ordered structure is even stronger than steel fibers of the same size. Collagen
makes bones strong but flexible. Collagen fibers in the skin’s dermis provide it with structure,
and the accompanying elastin protein fibrils make it flexible. Pinch the skin on your hand and
then let go; the collagen and elastin proteins in skin allow it to go back to its original shape.
Smooth-muscle cells that secrete collagen and elastin proteins surround blood vessels,
providing the vessels with structure and the ability to stretch back after blood is pumped
through them. Another strong, fibrous protein is keratin, an important component of skin,
hair, and nails.