Proteins and Nucleic Acids

BIO 351Human Anatomy and Physiology I

Proteins: Characteristics

Make up 10 to 30% of cell mass. Basic structural material of the body. Play vital roles in cell functioning. All contain carbon, hydrogen, oxygen,

and nitrogen. Some contain sulfur or phosphorus also.

All proteins are made up of building blocks called amino acids.

Muscle tissue contains the proteins myosin and actin.

Muscles

Amino Acids

20 commonly occur in nature, 8 of these are essential AA (can’t be synthesized by human body, must be included in diet)

Two important functional groups—the amino group (can act as a base and accept protons) and the carboxyl group (can act as an acid and donate protons).

All amino acids are identical except for the R group—the R groups are what make each amino acid different and chemically unique.

Basic Amino Acid Structure

R Groups Highlighted

Protein Formation

Proteins are formed from amino acids by dehydration synthesis.

The bonds between adjacent amino acids are called peptide bonds.

Most proteins contain over 100 amino acids and are truly macromolecules. Some contain up to 10,000 amino acids.

Organization of Proteins - 4 Structural Levels of Proteins

Primary structure– the linear sequence of amino acids in the chain.

Secondary structure—formed by coiling of the primary chain into an alpha helix (with hydrogen bonds maintaining the coiled structure) or a beta pleated sheet (hydrogen bonds hold primary polypeptide chains side by side in a pleated structure like an accordion).

Structural Levels of Proteins

Tertiary structure—achieved when an alpha helix or beta pleated sheet folds in a three dimensional way to produce a globular molecule.

The structure is maintained by both hydrogen and covalent bonds.

Structural Levels

Quaternary structure—happens when two or more polypeptide chains aggregate in a regular manner to form a complex protein.

The shape of a protein determines its function. Anything that causes the protein to unfold will result in the protein being unable to perform its job.

Hemoglobin

Fibrous Proteins

Strand-like Also called structural proteins. Most exhibit secondary structure only but some

have quaternary structure as well (collagen is an example of a protein with quaternary structure).

Insoluble in water Very stable Provide mechanical support and tensile strength. Examples include collagen and keratin (both of

which are present in skin), and the muscle proteins actin and myosin

Globular Proteins

Spherical and compact Tertiary structure; some with quaternary

structure as well Water soluble Chemically active Examples are enzymes and antibodies Also called functional proteins Susceptible to denaturing

Protein Denaturation

The activity of functional proteins depends on their three dimensional structure.

The hydrogen bonds responsible for maintaining the structure are fragile and can be broken by changes in both the physical and chemical environment.

Hydrogen bonds begin to break when the pH changes or the temperature rises above normal.

The proteins unfold and lose their biological activity.

Enzymes

Enzymes are globular proteins that act as biological catalysts.

Each enzyme is chemically specific. Enzymes work by lowering the

activation energy of a reaction. Enzymes act on substrates and trigger

chemical reactions in the body.

Enzyme Activity

Nucleic Acids

Contain carbon, hydrogen, oxygen, nitrogen, and phosphorus

Two types—DNA and RNA DNA is the genetic material of the cell and is

found in the nucleus. It replicates itself in order for cell division to occur. It provides instructions for protein synthesis.

There are 3 major types of RNA (messenger, transfer and ribosomal). Each has a different function.

DNA

DNA is coiled like a spiral staircase or ladder, known as a double helix

The 2 “backbones” are composed of alternating sugar and phosphate groups

Each “rung” of the ladder is composed of 2 nitrogenous bases hooked together by hydrogen bonds

Chromosomes are formed from DNA. Genes are sections of chromosomes. Taken together, all of the genetic material in a cell’s nucleus is known as a genome

Nucleotides

The structural units of nucleic acids Each nucleotide has three components

N-containing base 5-carbon sugar (a pentose) Phosphate group

(Note: A nucleoside consists of just a base plus a pentose sugar without the phosphate)

N-containing Bases

There are 5 different N-containing bases found in nucleic acids. These are divided into 2 types, purines and pyrimidines.

They follow these base-pairing rules: Adenine pairs with thymine Cytosine pairs with guanine Uracil replaces thymine in RNA Adenine and guanine are purines. They

each contain 2 rings. Cytosine, uracil, and thymine are

pyrimidines. They each have a single ring structure.

The nucleic acids are the largest molecules in the body.

DNA

DNA replicates itself before cell division and provides instructions for making all of the proteins found in the body.

The structure of DNA is a double-stranded polymer containing the nitrogenous bases A, T, G, and C, and the sugar deoxyribose.

Bonding of the nitrogenous bases in DNA is very specific; A bonds to T (via 2 hydrogen bonds), and G bonds to C (via 3 hydrogen bonds)

The bases that always bind together are known as complementary bases.

DNA polymerase III adds one nucleotide at a time to the 3’ end of the newly formed strand following base pairing rules

RNA

The major types of RNA are produced inside the nucleus, and then transported into the cytoplasm, where they are used to make proteins according to the instructions provided by the DNA.

The structure of most types of RNA is a single-stranded polymer containing the nitrogenous bases A (adenine), G (guanine), C (cytosine), and U (uracil), and the sugar ribose.

In RNA, G bonds with C, and A bonds with U.

Genetic Code

ATP

ATP is the energy currency used by the cell. ATP is an adenine-containing RNA

nucleotide that has two additional phosphate groups attached.

The additional phosphate groups are connected by high energy bonds.

Breaking the high energy bonds releases energy the cell can use to do work.