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Proteins

BCHM 211 Biochemistry 2 nd^ Semester AY 2020- Natural Science Department College of Arts and Sciences Our Lady of Fatima University /jlp

Proteins

• Proteins (Greek proteios, “primary” or “of first importance”) are biochemical molecules consisting of polypeptides joined by peptide bonds between the amino and carboxyl groups of amino acid residues.
  • Proteins perform a number of vital functions:
• Enzymes are proteins that act as biochemical catalysts.
• Many proteins have structural or mechanical functions (e.g., actin and myosin in muscles).
• Proteins are important in cell signaling, immune responses, cell adhesion, and the cell cycle.
• Proteins are a necessary component in animal diets.

Amino acids

• The amino acids are classified by the polarity of the R group side chains, and whether they are acidic or basic:
• neutral, nonpolar
• neutral, polar
• basic, polar (contains an additional amino group)
• acidic, polar (contains an additional carboxylate group)
• All of the amino acids are also known by a three letter and one-letter abbreviations.

Amino acids

• Since the amino acids (except for glycine) contain four different groups connected to the a-carbon, they are chiral, and exist in two enantiomeric forms:
• The amino acids in living systems exist primarily in the L form.

Amino acids: Zwitterions

• Lowering the pH of the solution causes the zwitterion to pick up a proton: Increasing the pH of the solution causes the zwitterion to lose a proton:

Amino acids: Zwitterions

• Since the pH of the solution affects the charge on the amino acid, at some pH, the amino acid will form a zwitterion. This is called the isoelectric point.
• Each amino acid (and protein) has a characteristic isoelectric point: those with neutral R groups are near a pH of 6, those with basic R groups have higher values, and those with acidic R groups have lower values.
• Because amino acids can react with both H 3 O+ and OH- , solutions of amino acids and proteins can act as buffers. (E.g., blood proteins help to regulate the pH of blood.)

Peptide formation

• Amides can be thought of as forming from the reaction of an amine and a carboxylic acid:
• In the same way, two amino acids can combine to form a dipeptide, held together by a peptide bond:

Peptides

• Short chains are referred to as peptides, chains of up to about 50 amino acids are polypeptides, and chains of more than 50 amino acids are proteins. (The terms protein and polypeptide are often used interchangeably.)
• Amino acids in peptide chains are called amino acid residues.
• The residue with a free amino group is called the N-terminal residue, and is written on the left end of the chain.
• The residue with a free carboxylate group is called the C-terminal residue, and is written on the right end of the chain.

Oxytocin and Vassopressin

• More than 200 peptides have been identified as being essential to the body’s proper functioning.
• Vasopressin and oxytocin are nonapeptide hormones secreted by the pituitary gland. Six of the amino acid residues are held in a loop by disulfide bridges formed by the oxidation of two cysteine residues. Even though the molecules are very similar, their biological functions are quite different:
  • Vassopressin is known as antidiuretic hormone (ADH) because it reduces the amount of urine formed, which causes the body to conserve water. It also raises blood pressure.
  • Oxytocin causes the smooth muscles of the uterus to contract, and is administered to induce labor. It also stimulates the smooth muscles of mammary glands to stimulate milk ejection.

Adrenocorticotropic hormone

• Adrenocorticotropic hormone (ACTH) is a 39-residue peptide produced in the pituitary gland. It regulates the production of steroid hormones in the cortex of the adrenal gland.

Characteristics of Proteins: Size

• Proteins are very large polymers of amino acids with molecular weights that vary from 6000 amu to several million amu.
• Glucose (C 6 H 12 O 6 ) = 180 amu
• Hemoglobin (C 2952

H

4664

O

832

N

812

S

8 Fe 4

65,000 amu

Characteristics of Proteins: Size

• Proteins are too large to pass through cell membranes, and are contained within the cells where they were formed unless the cell is damaged by disease or trauma.
• Persistent large amounts of protein in the urine are indicative of damaged kidney cells.
• Heart attacks can also be confirmed by the presence of certain proteins in the blood that are normally confined to cells in heart tissue.

Protein Function

• Proteins perform crucial roles in all biological processes.

1. Catalytic function: Nearly all reactions in living organisms are catalyzed by proteins functioning as enzymes. Without these catalysts, biological reactions would proceed much more slowly.
2. Structural function: In animals structural materials other than inorganic components of the skeleton are proteins, such as collagen (mechanical strength of skin and bone) and keratin (hair, skin, fingernails).
3. Storage function: Some proteins provide a way to store small molecules or ions, e.g., ovalbumin (used by embryos developing in bird eggs), casein (a milk protein) and gliadin (wheat seeds), and ferritin (a liver protein which complexes with iron ions).

Protein Function

4. Protective function: Antibodies are proteins that protect the body from disease by combining with and destroying viruses, bacteria, and other foreign substances. Another protective function is blood clotting, carried out by thrombin and fibrinogen.
5. Regulatory function: Body processes regulated by proteins include growth (growth hormone) and thyroid functions (thyrotropin).
6. Nerve impulse transmission: Some proteins act as receptors for small molecules that transmit impulses across the synapses that separate nerve cells (e.g., rhodopsin in vision).
7. Movement function: The proteins actin and myosin are important in muscle activity, regulating the contraction of muscle fibers.