Living systems use biological nanomotors to build life's essential molecules--such as DNA and proteins--as well as to transport cargo inside cells with both spatial and temporal precision. Each motor is highly specialized and carries out a distinct function within the cell. Some have even evolved sophisticated mechanisms to ensure quality control during nanomanufacturing processes, whether to correct errors in biosynthesis or to detect and permit the repair of damaged transport highways. In general, these nanomotors consume chemical energy in order to undergo a series of shape changes that let them interact sequentially with other molecules. Here we review some of the many tasks that biomotors perform and analyse their underlying design principles from an engineering perspective. We also discuss experiments and strategies to integrate biomotors into synthetic environments for applications such as sensing, transport and assembly.

Background The pathophysiology of hypertension in the immediate postpartum period is unclear. Methods and Results We studied 988 consecutive women admitted to a tertiary medical center for cesarean section of a singleton pregnancy. Angiogenic factors, soluble fms-like tyrosine kinase 1 (sFlt1) and placental growth factor (PlGF), both biomarkers associated with preeclampsia, were measured on antepartum blood samples. We then performed multivariable analyses to determine factors associated with the risk of developing postpartum hypertension. Of the 988 women, 184 women (18.6%) developed postpartum hypertension. 77 out of 184 women developed de novo hypertension in the postpartum period and the remainder had a hypertensive disorder of pregnancy in the antepartum period. A higher body mass index and history of diabetes mellitus were associated with development of postpartum hypertension. The antepartum sFlt1/PlGF ratio positively correlated with blood pressures in the postpartum period [highest postpartum systolic (r=0.29; P<0.001) and diastolic (r=0.28, P<0.001)]. Moreover, the highest tertile of the antepartum sFlt1/PlGF ratio was independently associated with postpartum hypertension [OR: 2.25 (1.19, 4.25), P=0.01 in the de novo hypertensive group and 2.61 (1.12, 6.05) in the persistent hypertensive group; P=0.02] in multivariable analysis. Women developing postpartum hypertension had longer hospitalization than those who remained normotensive (6.5 ± 3.5 versus 5.7 ± 3.4 days; P<0.001). Conclusions Hypertension in the postpartum period is relatively common and is associated with prolonged hospitalization. Women with postpartum hypertension share similar clinical risk factors as well as a similar antepartum plasma angiogenic profile found in women with preeclampsia. These data suggest that postpartum hypertension may represent a group of women with subclinical or unresolved preeclampsia.

Coumarins have attracted intense interest in recent years because they have been identified from natural sources, especially green plants and have diverse pharmacological properties. In this study, we investigated whether 7,8-dihydroxy-4-methylcoumarin (DHMC) caused apoptosis in A549 human non-small cell lung carcinoma cells (NSCLC) and, if so, by what mechanisms. Here, we show that, in A549 human NSCLC cells, DHMC induces apoptosis through mitochondria-mediated caspase-dependent pathway. Although an increase in the levels of reactive oxygen species (ROS) was observed, pre-treatment with antioxidant showed no protective effect against DHMC-induced apoptosis. In addition, our immunoblot data revealed that DHMC treatment led to down-regulation of Bcl-xl, Bax, p21, Cox-2, p53 and upregulation of c-Myc. Results in the present study for the first time suggest that DHMC induces apoptosis in human lung A549 cells through partial inhibition of ERK/ MAPK signaling.

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Recent experiments have measured the rate of replication of DNA catalyzed by a single enzyme moving along a stretched template strand. The dependence on tension was interpreted as evidence that T7 and related DNA polymerases convert two (n ‫؍‬ 2) or more single-stranded template bases to double helix geometry in the polymerization site during each catalytic cycle. However, we find structural data on the T7 enzyme-template complex indicate n ‫؍‬ 1. We also present a model for the ''tuning'' of replication rate by mechanical tension. This model considers only local interactions in the neighborhood of the enzyme, unlike previous models that use stretching curves for the entire polymer chain. Our results, with n ‫؍‬ 1, reconcile force-dependent replication rate studies with structural data on DNA polymerase complexes.T he advent of techniques to micromanipulate single molecules (1, 2) has enabled studies of DNA elasticity (3-6) and the kinetics of motor enzymes (7-11). Applying force either to a tethered enzyme or to the substrate polymer is often found to markedly alter enzyme-catalyzed rates and thereby offers insight into conformational changes involved in operation of the molecular motor. We consider such results pertaining to the rate at which DNA polymerases, operating on a stretched single-strand (ss) DNA template, catalyze synthesis of a complementary strand (10, 11). The original interpretation of the data concluded that in the polymerization site of the motor enzyme n ϭ 2 or even n ϭ 4 (depending on the enzyme) ss template bases are converted to double-stranded (ds) geometry during each catalytic cycle, only to have n Ϫ 1 of these bases revert to ss geometry before the onset of the next cycle. If correct, this conclusion would have important implications for the mechanisms of DNA replication, proofreading, and editing (15,16).Crystal structures of enzyme-DNA complexes indicate, however, that only one template base is converted from ss to ds geometry in the complex (12,13,(17)(18)(19). We attribute this apparent conflict to misleading aspects of the previous models (10, 11) used to interpret the force dependence of the replication rate. We also suggest conceptual amendments that indicate the rate data are compatible with n ϭ 1, in accord with the structural results. Fig. 1 depicts schematically the elementary rate-limiting step thought to govern DNA replication (12,(14)(15)(16). This process involves a change in the conformation of the DNA bound to the enzyme, in which the leading base of the ssDNA template strand (labeled 0) pairs with a complementary dNTP that is incorporated into the growing double helix. A key aspect affecting the response to tension applied to the template is the change in length that occurs during conversion of ss-to dsDNA. This change is specified in terms of a decrease from L ss to L ds , the corresponding contour lengths per residue. Operating as a molecular motor to generate mechanical force from chemical energy, DNA polymerase (DNAp) induces this shrinkage in successive steps as ...

Molecular dynamics simulations are presented for a Thermus aquaticus (Taq) DNA polymerase I complex (consisting of the protein, the primer-template DNA strands, and the incoming nucleotide) subjected to external forces. The results obtained with a force applied to the DNA template strand provide insights into the effect of the tension on the activity of the enzyme. At forces below 30 pN a local model based on the parameters determined from the simulations, including the restricted motion of the DNA bases at the active site, yields a replication rate dependence on force in agreement with experiment. Simulations above 40 pN reveal large conformational changes in the enzyme-bound DNA that may have a role in the force-induced exonucleolysis observed experimentally.

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Recent single-molecule experiments reveal that mechanical tension on DNA can control both the speed and direction of the DNA polymerase motor. We present a theoretical description of this tension-induced ''tuning'' and ''switching.'' The internal conformational states of the enzyme motor are represented as nodes, and the allowed transitions between states as links, of a biochemical network. The motor moves along the DNA by cycling through a given sequence of internal states. Tension and other external control parameters, particularly the ambient concentrations of enzyme, nucleotides, and pyrophosphates, couple into the internal conformational dynamics of the motor, thereby regulating the steady-state flux through the network. The network links are specified by bulk-phase kinetic data (in the absence of tension), and rudimentary models are used to describe the dependence on tension of key links. We find that this network analysis simulates well the chief results from single-molecule experiments including the tension-induced attenuation of polymerase activity, the onset of exonucleolysis at high tension, and insensitivity to large changes in concentration of the enzyme. A major dependence of the switching tension on the nucleotide concentration is also predicted. Single-molecule experiments (1-3) enable the velocity of a polymerase motor to be monitored as it catalyzes the replication of a mechanically stretched DNA template. Fig. 1 shows results for the widest range of tension currently available (2). Applying piconewton forces to the DNA template markedly alters (''tunes'') the rate of replication catalyzed by T7 DNA polymerase (DNAp). Stretching the DNA with tension greater than Ϸ35 pN induces the enzyme motor to reverse (''switch'') directions and begin depolymerizing the double-stranded (ds) DNA. Previous theoretical models (2-6) have related the tension dependence of the replication rate to free-energy changes but have not accounted for the exonucleolysis process that occurs above 35 pN. This process is of particular interest, because its mechanism may prove to be akin to the proofreading or editing function of the enzyme, which is triggered by a mismatch in DNA base pairing.This article treats both the polymerase and exonucleolysis modes by means of a pragmatic modeling procedure based on the analysis of reaction cycles extensively developed by Hill (7). This offers a systematic way to incorporate structural, thermodynamic, and kinetic data from bulk as well as single-molecule experiments and to examine assumptions pertaining to the effect of tension or other control variables on enzyme action. Aspects of the results are chiefly heuristic but serve both to suggest experimental tests and to aid the design of molecular dynamics simulations (6). Kinetic Network AnalysisThe kinetic pathway (8-10) for T7 DNAp shown in Fig. 2A can be described by the cyclic network of Fig. 2B; the nodes (i ϭ 1-7) of the network represent known states of the enzyme-DNA complex, and the links specify transitions with the rate const...