Protein Classification Based on Function

Question 1

Proteins are probably best known for their role as catalysts. Proteins with the role of biochemical catalyst are called enzymes. Enzymes participate in almost all of the metabolic reactions
that occur in cells.

Answer 1

Catalytic proteins

Question 2

These proteins, also called immunoglobulins or antibodies, are central to the functioning of the body’s immune system. They bind to foreign substances, such as bacteria and viruses, to help combat invasion of the body by foreign particles.

Answer 2

Defense proteins

Question 3

These proteins bind to particular small biomolecules and transport them to other locations in the body and then release the small molecules as needed at the destination location. The most well-known example of a ________ is hemoglobin, which carries oxygen from the lungs to other organs and tissues. Another transport protein is transferrin, which carries iron from the liver to the bone marrow. High- and low-density lipoproteins are carriers of cholesterol in the bloodstream.

Answer 3

Transfer proteins

Question 4

These proteins transmit signals to coordinate biochemical processes between different cells, tissues, and organs. A number of hormones
that regulate body processes are messenger proteins, including insulin and glucagon. Human growth hormone is another example of a ______.

Answer 4

Messenger proteins

Question 5

These proteins are necessary for all forms of movement. Muscles are composed of filament-like contractile proteins that, in response to nerve stimuli, undergo conformation changes that involve contraction and extension. Actin and myosin are examples of such proteins. Human reproduction depends on the movement of sperm. Sperm can “swim” because of long flagella made up of __________.

Answer 5

Contractile proteins

Question 6

These proteins confer stiffness and rigidity to otherwise fluid-like biochemical systems. Collagen is a component of cartilage, and a keratin gives mechanical strength as well as protective covering to hair, fingernails, feathers, hooves, and some animal shells.

Answer 6

Structural proteins

Question 7

These proteins, which span a cell membrane, help control the movement of small molecules and ions through the cell membrane. Many such proteins have channels through which molecules can enter and exit a cell. Such protein channels are very selective, often allowing passage of just one type of molecule or ion.

Answer 7

Transmembrane proteins

Question 8

These proteins bind (and store) small molecules for future use. During degradation of hemoglobin the iron atoms present are released and become part of ferritin, an iron-storage protein, which saves the iron for use in the biosynthesis of new hemoglobin molecules. Myoglobin is an oxygen-storage protein present in muscle; the oxygen so stored is a reserve oxygen source for working muscle.

Answer 8

Storage proteins

Question 9

These proteins are often found “embedded” in the exterior surface of cell membranes. They act as sites at which messenger molecules, including messenger proteins such as insulin, can bind and thereby initiate the effect that the messenger “carries.” _____________ are often the molecules that bind to enzymes (catalytic proteins), thereby turning them “on” and “off” and thus controlling enzymatic action

Answer 9

Regulatory proteins

Question 10

These proteins are particularly important in the early stages of life, from embryo to infant. Casein, found in milk, and ovalbumin, found in egg white, are two examples of such proteins. The role of milk in nature is to nourish and provide immunological protection for mammalian
young. Three-fourths of the protein in milk is casein. More than 50% of the protein in egg white is ovalbumin.

Answer 10

Nutrient proteins

Question 11

These proteins are part of the system by which the acid-base balance within body fl uids is maintained. Within the blood, the protein hemo-
globin has a buffering role in addition to being an oxygen carrier. Transmembrane proteins regulate the movement of ions in and out of cells, ensuring that ion concentrations are those needed for correct acidity/alkalinity.

Answer 11

Buffer proteins

Question 12

These proteins help maintain fluid balance between blood and surrounding tissue. Two well-known __________, found in the capillary beds of the circulatory system, are albumin and globulin. When increased blood pressure generated by a pumping heart forces water and nutrients out of the capillaries, these proteins remain behind (since they are too big to cross cellular membranes). As their concentration increases (due to less fluid being present), osmotic pressure “forces” draw water back into the capillaries, which is necessary for fluuid balance to be maintained.

Answer 12

Fluid-balance proteins