Lecture #2: Review of the Chemistry of Life (cont'd)

IV. PROTEINS (CHON + S) (means "first place" after Greek word "proteios")

A. PRIMARY STRUCTURE (the basic sequence of amino acids) (20 AAs)

1. Composed of simple building blocks (monomers) called amino acids. (Fig 5.15)

2. AAs link together by peptide bonds to form polypeptide chains (polymers). (Fig 5.16)

3. Proteins, per se, are usually about 40-500 AAs.

4. A 100-AA chain has the possibility of 20¹⁰⁰ sequences (20¹⁰ = 10,240,000,000,000).

5. Lysozyme (denatures bacterial walls) has 129 AAs. (Fig 5.18)

B. SECONDARY STRUCTURE (due to hydrogen bonds formed at regular intervals)

1. AAs are not straight, but coiled strands, due to H₂ bonds, called alpha helixes. (Fig 5.20)

2. Fibrous proteins usually have helical patterns. (keratins form nails, hooves, horns)

C. TERTIARY STRUCTURE (when polypeptide chains fold into globular forms)

1. Wherever there is a proline, a kink or bend occurs in the chain.

2. Disulfide bonds and other forces cause bending. (cysteine) (Fig 5.22)

3. Globular proteins have tertiary structure. (enzymes, proteinaceous hormones, antibodies, albumin, gamma globulin, and most blood proteins)

D. QUATERNARY STRUCTURE (when two or more polypeptide subunits aggregate)

1. Hemoglobin, for example, is composed of 4 globular proteins. (Fig 5.23)

2. Collagens from fibroblast cells:

a. collagen fibrils consist of a triple helix (i.e., three alpha helixes together).

b. constitute 1/3 of body protein in mammals.

c. in skin, tendons, ligaments, bones, cornea, vessels, etc.

d. possess the tensile strength of cast iron.

E. SUMMARY OF PROTEIN STRUCTURE (Fig 5.24)

F. OTHER CHARACTERISTICS OF PROTEINS

1. Conjugated proteins have sugar, or other carbon or lipid entities (e.g., hCG)

2. Denatured proteins result from temps above 60°C, acids, alkali, or salts. (Fig 5.25)

G. PROTEIN FUNCTIONS (You can see Table 5.1)

1. They provide structure and support in connective tissue, muscle, bone, etc.

2. They provide storage of amino acids (ovalbumin).

3. They transport (and protect) other substances (hemoglobin-O₂; binding proteins).

4. They act as chemical messengers to coordinate bodily activities (hormones).

5. They make up receptors and ion channels that characterize plasma membranes.

6. They promote movement by interacting to cause contraction of cells (actin-myosin).

7. They serve as defense mechanisms in the form of antibodies.

8. They form enzymes for chemical control of metabolism and homeostasis.

V. NUCLEIC ACIDS (Fig 5.29)

1. Composed of monomers called nucleotides (pentose + phosphate + nitrogenous base)

2. DNA versus RNA:

a. the sugar in RNA is ribose, instead of deoxyribose.

b. RNA contains uracil, in place of thymine.

c. RNA is single-stranded, instead of double-stranded. (Fig 5.30)

3. Functions are genetic transmission, and genetic coding of AA sequence in proteins.

4. DNA® mRNA® protein: a diagrammatic overview of information flow in a cell (Fig. 5.28)

5. Discovery of DNA and protein sequences by the techniques of molecular biology has added a new method of commensurating (i.e., measuring) evolutionary kinship of different organisms. (Table 5.2)

(6. Adenosine triphosphate (Fig 6.8)

A BRIEF REVIEW OF CELLS AND CELL MEMBRANES

I. CELLS

1. A "typical" textbook picture of an animal cell. (Fig 7.7)

2. The cell surface is usually covered by a coat of material secreted by the cell.

a. the cell wall of plants is a secretion of the polysaccharide cellulose.

b. animal cells may be coated with sticky oligosaccharides called glycocalyx.

3. Animal cells are connected by tight junctions, desmosomes, and gap junctions.

Plant cells are basically the same, except for chloroplasts and a vacuole. (Fig 7.8)

II. CELL MEMBRANES

1. The plasma membrane is the boundary between cells and their surroundings.

2. Thus, the plasma membrane must have the capacity to control what goes in and out.

3. The FLUID-MOSAIC MODEL of a cell membrane:

a. It is based on a bilayer of phospholipids (which naturally form). (Fig 8.2)

b. The lipid molecules can move (drift) laterally around the membrane.

c. Proteins are inserted within this bilayer of phospholipids. (Fig 8.6)

d. The proteins determine most of the specific functions of the membrane.

(1) transport ions by ion pumps and ion gates

(2) comprise enzymes that carry out membrane functions

(3) serve as receptors for hormones, etc. (Fig 8.7)

(4) provide cell-to-cell recognition (e.g., between gametes)

(5) make intercellular junctions (e.g., desmosomes)

(6) provide attachments to cytoskeleton and extracellular matrix

(7) transport proteins (not soluble in lipids) by endocytosis and exocytosis

III. NUCLEAR MEMBRANES (Fig. 7.9)

1. A double membrane, each consisting of a lipid bilayer (with proteins).

2. The membrane is perforated by pores about 100 nm in diameter.

3. The most visible nucleolus (or, nucleoli) synthesize ribosomes.

4. Ribosomes pass thru pores to cytoplasm where they are assembled.

5. DNA transcribes mRNA which is transported to ribosomes for translation.

6. Translation products include: a. free (cytosolic) ribosomes, which usually make enzymes. b. bound (to ER) ribosomes, which usually make proteins, package proteins (inside lysosomes), or make proteins to be secreted (e.g., hormones).

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