What are 5 functions of proteins?

Learning Outcomes

• Identify several major functions of proteins

The primary types and functions of proteins are listed in Table 1.

Two special and common types of proteins are enzymes and hormones. Enzymes, which are produced by living cells, are catalysts in biochemical reactions (like digestion) and are usually complex or conjugated proteins. Each enzyme is specific for the substrate (a reactant that binds to an enzyme) it acts on. The enzyme may help in breakdown, rearrangement, or synthesis reactions. Enzymes that break down their substrates are called catabolic enzymes, enzymes that build more complex molecules from their substrates are called anabolic enzymes, and enzymes that affect the rate of reaction are called catalytic enzymes. It should be noted that all enzymes increase the rate of reaction and, therefore, are considered to be organic catalysts. An example of an enzyme is salivary amylase, which hydrolyzes its substrate amylose, a component of starch.

Hormones are chemical-signaling molecules, usually small proteins or steroids, secreted by endocrine cells that act to control or regulate specific physiological processes, including growth, development, metabolism, and reproduction. For example, insulin is a protein hormone that helps to regulate the blood glucose level.

Proteins have different shapes and molecular weights; some proteins are globular in shape whereas others are fibrous in nature. For example, hemoglobin is a globular protein, but collagen, found in our skin, is a fibrous protein. Protein shape is critical to its function, and this shape is maintained by many different types of chemical bonds. Changes in temperature, pH, and exposure to chemicals may lead to permanent changes in the shape of the protein, leading to loss of function, known as denaturation. Different arrangements of the same 20 types of amino acids comprise all proteins. Two rare new amino acids were discovered recently (selenocysteine and pyrrolysine), and additional new discoveries may be added to the list.

Proteins are a class of macromolecules that perform a diverse range of functions for the cell. They help in metabolism by providing structural support and by acting as enzymes, carriers, or hormones. The building blocks of proteins (monomers) are amino acids. Each amino acid has a central carbon that is linked to an amino group, a carboxyl group, a hydrogen atom, and an R group or side chain. There are 20 commonly occurring amino acids, each of which differs in the R group. Each amino acid is linked to its neighbors by a peptide bond. A long chain of amino acids is known as a polypeptide.

Proteins are organized at four levels: primary, secondary, tertiary, and (optional) quaternary. The primary structure is the unique sequence of amino acids. The local folding of the polypeptide to form structures such as the α helix and β-pleated sheet constitutes the secondary structure. The overall three-dimensional structure is the tertiary structure. When two or more polypeptides combine to form the complete protein structure, the configuration is known as the quaternary structure of a protein. Protein shape and function are intricately linked; any change in shape caused by changes in temperature or pH may lead to protein denaturation and a loss in function.

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Each of us has tens of thousands of proteins, which serve a variety of functions, and each protein has a unique three-dimensional structure that specifies its function. For example, hemoglobin

from http://gened.emc.maricopa.edu/bio/bio181/BIOBK/3_14d.jpg

Sickle Cell Anemia The critical relationship between protein structure and function is dramatically illustrated by sickle cell anemia, an inherited disease seen in people whose ancestors came from Africa, the Middle East, the Mediterranean, or India. In the U.S. about 4 out of every 1,000 African Americans has sickle cell disease (about 80,000 people), and about 10% carry the sickle cell trait. People with sickle cell disease an abnormal type of hemoglobin, called hemoglobin S (instead of normal hemoglobin A). Hemoglobin S differs from hemoglobin A in that the amino acid valine is found at position number 6 in the beta chain instead of the amino acid glutamate. Unlike glutamate, the side chain of valine is very non-polar and creates a sticky patch on the outside of each of the beta chains. There is a complementary sticky patch elsewhere on the hemoglobin, but it is masked as long as the hemoglobin molecules are bound to oxygen. However, if large numbers of hemoglobin molecules become deoxygenated, the sticky sites created by the abnormal valines begin to bind to the complementary sticky site on other hemoglobin molecules. This forms long aggregates of hemoglobin that distort the red blood cell and give it a characteristic sickle shape. This causes red cells to aggregate and impairs their ability to circulate through small blood vessels (arterioles and capillaries), and it also makes them fragile, shortening their life span and leading to anemia.   Acute exacerbations referred to as "sickle cell crises" are sometimes triggered by deoxygenation of hemoglobin, for example, after rigorous exercise or infections. Sickling can be very extensive and result in inadequate blood flow to organs with severe pain and complications such as stroke, kidney damage and breathing problems.

Proteins as Enzymes

Some proteins function as enzymes, i.e., proteins that catalyze specific biochemical reactions. Enzymes facilitate biochemical reactions and speed them up enormously, making them as much as a million times faster. There are thousands of enzymes, and each type facilitates a specific biochemical reaction. In other words, a given enzyme only acts on specific reactant molecules (substrates) to produce a specific end product or products. The diagram below illustrates enzymatic cleavage of the disaccharide lactose (the substrate) into the monosaccharides galactose and glucose.

Source: http://www.indiana.edu/~ensiweb/lessons/tp.milk3.html 

The three-dimensional shape of an enzyme will include a very specific binding site that the substrate will fit into very precisely, in much the same way that a key fits a specific lock.

Once the substrate is bound the enzyme cleaves the substrate and the products are released. While this cartoon illustrates cleavage of a substrate, many enzymes synthesis new biochemicals by binding two substrates together to form a new product. A particular cell may have thousands of distinct enzymes catalyzing many different reactions.

The short video below illustrates the basics of how an enzyme works.

Sources: http://youtu.be/V4OPO6JQLOE

Biochemical reactions may require a whole series of steps, each of which is catalyzed by a separate enzyme. A good example is the series of reactions by which glucose is metabolized to create cellular energy in the form of ATP (adenosine triphosphate).

• First there is a series of reactions that convert glucose into two molecules of pyruvate as shown below. These reaction are referred to as "glycolysis" or as anaerobic metabolism (since they don't require oxygen). There is net production of a small amount of ATP from these reactions.
• Then, pyruvate then feeds into the citric acid cycle (also known as the Kreb's cycle or tricarboxylic acid cycle). The subsequent metabolism of pyruvate generates even more net ATP than glycolysis through the reaction illustrated below. The steps highlighted in yellow are the ones that ultimately go on to generate ATP in the cell's mitochondria.

These reactions are illustrated in the figure below.

Source: http://chemwiki.ucdavis.edu/Core/Biological_Chemistry/Metabolism/Kreb's_Cycle

Lysozyme - A Defensive Enzyme

The illustration on the right shows the protein lysozyme

Antibodies are Proteins

Watch the short video below to see an illustration of antibody action. The beginning of the video shows red and white blood cells flowing through a blood vessel. The potato-shaped objects that you see next represent viruses that begin binding to receptors on a cell. The green Y-shaped objects represent antibodies that bind to the virus. Finally, the Medusa-like structure represents a white blood cell that engulfs the antibody-tagged virus and destroys it.

Structural Proteins

There are also structural proteins, which are frequently long and fibrous, such as silk, keratin in hair, and collagen in tendons and ligaments.

Source: http://www.sdsc.edu/ScienceAlive/reel6/collagen.gif

Contractile Proteins

There are contractile proteins, such as actin and myosin, that provide movement in muscles and movement within single cells.

Source: http://www.bmb.psu.edu/courses/bisci004a/muscle/musc-img/myofibril.jpg

Signal Proteins

 See http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/pancreas/insulin_phys.html  for a Flash model on insulin action.

Transportation Across the Cell Membrane

With the exception of simple diffusion, proteins are also essential for moving polarized or charged molecules and large molecules across cell membranes.

Simple Diffusion

Small molecules like oxygen and carbon dioxide can diffuse across the lipid bilayer of the cell membrane. The direction of movement depends on the concentration gradient. Substances with higher concentration inside the cell (e.g., CO2) will diffuse out of the cell toward the side with lower concentration. Substances in higher concentration outside the cell (e.g., O2) will diffuse to the inside of the cell, i.e., down the concentration gradient.

However, many other molecules cannot cross cell membranes by simple diffusion and require specialized mechanisms for movement across membranes. A variety of transport proteins, frequently aggregates of protein subunits, provide a way of transporting charged molecules and large molecules through one of two mechanisms:

Facilitated Transport

Polar molecules and charged ions cannot cross the lipid bilayer; their transit relies on special transport channels created by proteins embedded in the cell membrane. Facilitated transport is passive in that it does not require expenditure of cellular energy, and as with simple diffusion, movement of the molecules is down a concentration gradient from high concentration to low concentration. There are specific proteins for each substance transported by this mechanism, and transit can be regulated by the cell. Molecules like glucose and amino acids are transported this way. They will bind to their carrier/transpport protein, and binding triggers a change in the shape of the carrier which moves the molecule across the membrane. Once the molecule is released, the carrier returns to its original shape (conformation).

Active Transport

Active transport also relies on transmembrane transport proteins, but this process is able to transport substances against a conentration gradient, meaning that even if the concentration of, say potassium ions, is higher inside the cell than outside, more potassium can be transported into the cell. This is because cellular energy (ATP) is expended.

Proteins, then, play an integral role in the function of a cell. Many are embedded in the cell's membranes or span the entire lipid bilayer where they play an important role in recognition, signaling, and transport.